what appears to be the English/American version of the intro.
I almost wonder if this can really be the original intro, or if it's consistently mislabeled, and this is for season three or something instead?
The links you supplied are for the Canadian Sailor Moon opening animation sequence and for the Pretty Soldier Sailor Moon (Bishoujo Senshi Sailor Moon [『美少女戦士セーラームーン』] [far later, the official English translation of the franchise changed to Pretty Guardian Sailor Moon]) Season 1 first opening animation sequence (out of 3 opening animations for that season).
I'd love to hear if there are any interviews or reliable sources discussing what could've justified this decision to so completely change the opening theme and intro cut.
Rather than seek an interview or statement from the companies involved that addresses their thought process on the opening sequence, it may be helpful to consider the historical background of the decision...
Not many anime series had made it to American broadcast television at the time that Bishoujo Senshi Sailor Moon [『美少女戦士セーラームーン』) was picked up for adaptation into Sailor Moon, which was dubbed and aired in Canada before subsequent export to the United States. Almost all of the anime that had previously aired were heavily edited shounen or children's anime, such as the largely re-written mash-up of the 3 Japanese series Super Dimension Fortress Macross (Choujikuu Yousai Macross [『超時空要塞マクロス』]) + The Super Dimension Cavalry Southern Cross (Choujikuu Kidan Sazan Cross [『超時空騎団サザンクロス』]) + Genesis Climber MOSPEADA (Kikou Souseiki MOSPEADA [『機甲創世記モスピーダ』]) into 1 American series called Robotech, or cutting out content viewed as inappropriate for children in Noozles (originally, Mysterious Koala Blinky [Fushigi na Koala Blinky, 『ふしぎなコアラブリンキー』]). Mighty Morphin Power Rangers had used footage from the live-action sentai (superhero team) TV series Dinosaur Squad Beast-ranger (Kyouryuu Sentai Zyu-ranger,『恐竜戦隊ジュウレンジャー』) for all scenes in which characters were in battle costume (helmets covering their faces) but replaced the Japanese cast of the characters' un-transformed civilian mode with completely different characters of other ethnicities who lived in a completely different setting. The goal in every case was to make the cost of dubbing and broadcast rights worth it, by appealing enough to what the anticipated American child audience would be interested in and avoiding angering parents who might boycott the product, so as to at least making enough money to break even, but preferably to be able to sell merchandise to profit from it.
In contrast to these, the Canadian Sailor Moon broadcast took three main risks. 1) It did not include any main male characters, so it was not expecting to appeal to boys. (An example of a company attempting to mitigate this risk is the later American broadcast of Card Captors, an adaptation of Cardcaptor Sakura [『カードキャプターさくら』] heavily-edited to make it seem to be a team fighting story focusing a disproportionate amount of screen time on Li Syaoran compared to his smaller-scope role in the original Japanese series.) 2) Furthermore, when Sailor Moon aired in America, it was very different from what American girls or even Japanese girls had been used to seeing: a group of superhero girls who fight as a team against evil forces threatening the Earth. While many people who discovered the mahou shoujo genre (of any female character who has magical powers while living in a non-magical environment) either through or after Bishoujo Senshi Sailor Moon are unaware of the fact, the Bishoujo Senshi Sailor Moon manga pioneered sentai-style mahou shoujo (magical girl) within the mahou shoujo genre; before this series, the history of magical girls in manga and anime usually used their powers for nichijou (daily life) matters or for moonlighting as an idol singer rather than for protecting the world against evil powers, and they did not cooperate with a team of comrades (some of the primary examples include Magical Mako-chan [Mahou no Mako-chan,『魔法のマコちゃん』], Witch Meg [Majokko Meg-chan,『魔女っ子メグちゃん』], Witch Sally [Mahoutsukai Sally,『魔法使いサリー』], Magic Angel Creamy Mami [Mahou no Tenshi Creamy Mami,『魔法の天使クリィミーマミ』]), Magical Princess Minky Momo [Mahou no Princess Minky Momo『魔法のプリンセス ミンキー モモ』], Akko-chan's Secret [Himitsu no Akko-chan, 『ひみつのアッコちゃん』], and Legendary Idol Eriko [Idol Densetsu Eriko, 『アイドル伝説えり子』). Even the outlier non-shoujo mahou shoujo series that were targeted at Japanese male viewers rather than at Japanese girls, such as ESPer Mami (『エスパー魔美』) and Cutie Honey (『キューティーハニー』 [though its post-Sailor Moon version, Cutie Honey Flash [『キューティーハニー フラッシュ』], was an actual shoujo series with a target demographic of young girls]), did not have teams of girls or super-powered villains.
At that time, American animated TV series for girls were usually about a single girl or animal in slice-of-life settings, or, even if she had an ensemble cast like Rainbow Brite or My Little Pony Tales, they did not usually fight battles as a team (though not superhero/sentai in style, notable exception to this are the female groups in Jem and Lady Lovely Locks). 3) As a third risk, the personality of Usagi (in the North American dub, Serena) was also rather different from audience expectations: while Americans had seen cluelessness/laziness/gluttony and perhaps even incompetence in male protagonists, such as in Inspector Gadget or Scooby Doo, this sort of female lead character was not at all common.
Why did they so heavily change the already perfect intro sequence and opening theme like that?
the entire re-cut American/English one, which also has changed the theme song entirely.
First, the opening theme song with its selected corresponding animation uses the same basic music as the original Japanese theme song, a rather unusual move for English dubs in that period. While the English Dragon Ball had done so from the Japanese Dragonball, these were not the majority of cases. For contrast, see the openings of English Noozles vs. Japanese Fushigi na Koala Blinky, English Escaflowne vs Japanese Escaflowne of the Heavens (Tenkuu no Escaflowne, 『天空のエスカフローネ』), and English Cardcaptors vs. Japanese Cardcaptor Sakura.
Second, to call the first Japanese opening "already perfect" is inaccurate...
The first Japanese opening animation only includes Sailor Moon/Usagi, Luna, Sailor Mercury/Ami, Sailor Mars/Rei, Tuxedo Kamen (civilian Mamoru is excluded), Beryl, and generic youma standing behind Beryl (who do not match any of the distinctive youma designs we see in the actual series). This first opening was clearly intended to be the opening animation only until the point in between when Mercury debuted (episode 8) and when Mars appeared (episode 10). The fabric covering both the silhouettes of Mercury and Mars in one shot provide an intentional teaser... but, interestingly, they are also shown walking in full-color within the same opening, and then the series switched to the second opening before the episode in which Mars/Rei first appears.
That first opening animation, which was only used for 20% of the length of the first season, was replaced early on by a minor change version in which the fabric covering Mercury and Mars was lifted: the second opening (another useful example of this intentional replacement of opening animation to the same song was used in Season 5 to create a teaser for the Sailor Starlights; you can see both versions side-by-side here). By excluding Sailor Jupiter/Makoto, Sailor Venus/Minako, and Artemis, this second opening of Season 1 was also clearly designed to be replaced at a later point rather than viewed by the animators as the standard for the season. Also, against the idea of this opening being "already perfect," it utilizes many still pans rather than moving animation, and uses up (arguably, wastes) a lot of its screen time on off-topic content that is not part of the story in any way: either a cartoonish cityscape or amusement park, roses unfurling one after another in 8 different colors, shots of masks unlike the one Tuxedo Kamen actually wears, and a silhouette of Usagi standing next to a fountain under a dirigible airship facing the silhouette of what appears to be a man with hair longer than Mamoru's but shorter than Nephrite's (clearly, whoever he is, he isn't Mamoru/Tuxedo Kamen/Endymion). This second opening animation was used until after Jupiter/Makoto appeared (episode 24 - about halfway through the series).
The third opening (note that the linked YouTube video title mistakenly labels this as the second opening, though it is actually the third) of Season 1 was comprised of completely different animation; included Serenity, Endymion, Jupiter/Makoto, and Venus/Minako; and devoted almost all of its screen time to on-topic content (the off-topic being Tuxedo Kamen riding a flying horse). However, it deleted any inclusion of Luna and Beryl (despite Luna being a major character with more Season 1 appearances than any other character except for Usagi herself!) and still lacked a good number of characters who would have contributed to a more representative opening if they had been included even for half a second: Mamoru, Motoki, Naru, Umino, Haruna-sensei, Artemis, the Shitennou ([Four Heavenly Kings, 四天王]: Jedite, Nephrite, Zoicite, Kunzite), the Saikyou Youma Nananinshu (Seven Strongest Youma, [最強妖魔七人衆]), "dark" Endymion, and Queen Metalia (contrast this with the contemporaneous Magic Knight Rayearth opening sequence, which capably included all main villains and allies). This third opening animation was used until the end of the first season.
Not only does it contain a weird intro part not even seen elsewhere, but it announces main characters which only join very late in the series, right away in the intro for the first episode.
Wasn't the point of importing this very Japanese show to the West to keep the look, sound and atmosphere, rather than changing it all around? I mean, wasn't there already a wealth of American-origin animated shows? Why did they have to change around Sailor Moon like this?
Even if the answer is that it had to attract a very different audience who expects things to be a certain way, it still doesn't explain why they would introduce characters from far into the series right away.
As explained above, the purpose of the Canadian dub aligned with the purposes of other anime dubs up until and past that time, but because the show aimed to pick up female viewers familiar with Mighty Morphin Power Rangers, using the Japanese opening animation that had been only used in the Japanese series' first 9 episodes would have failed to convey that this is a sentai-style team fighting story. It would have been possible for the dub to use the Season 1 third opening toward this end, but the first and second Japanese opening would have obfuscated this point. Unlike the Japanese broadcast, the English dub could not wait until 70% of the way through the season to switch over to an opening sequence that showed all five sailor soldiers, because it had no guarantee of the show not getting cancelled well before then (in the United States, it was not aired in a prime time slot or even at a time of day that most children were available to watch it even if they wanted to). Likely, making three consecutive opening animation sequences to align with the one English song would have cost additional time/money to no clear benefit.
In addition, all-American Boy Scouting and Girl Scouting had a clean and positive image, so the dub changed "sailor soldiers" to "sailor scouts," and the selection of scenes used for the dub's opening sequence included a shot of Naru during the lyrics, "She will never turn her back on a friend," a value that aligns with scouting (which is adventurous but much more about functioning well in daily-life than fighting enemies). If any of the Japanese opening sequences had included Naru, the dub might have utilized those animation cels of Naru, but they did not exist.
I'm not sure, but I believe that they didn't touch the show itself. (Perhaps they did, though. I can imagine that certain shots where you see the underpants of the girls when they jump around might have been cut away.)
While nowhere near the level of cut-and-paste as Robotech's, the Canadian dub did make a good number of edits, which you can read more about on Anime & Manga StackExchange here: