So as you can see in the fillers, that when Ashura is helping out the other villagers, he is using a jutsu just like the Rasengan.
It depends on what you consider canon. Almost all of the Ashura and Indra and even Kaguya backstory stuff was anime only filler. The manga has almost none of that. Thus, in the manga, there was no mention of Ashura ever using Rasengan, or pretty much any specific jutsu for that matter. Indra's eyes barely resembled a sharingan, and Hagoromo was never known to not have the Rinnegan. So in that regard, Ashura isn't the creator, its just anime only filler. They likely chose things like the Rasengan and Sasuke's Mangekyou and put them on the two brothers to try and make it all seem more like destiny or something, symbolism that things are repeating themselves. Its pretty low quality story and a plot hole because Madara doesn't have the same pattern even though he was also an Indra trans-migrant. The story wasn't about destiny, but instead about how Naruto used the power he was given to change things, when the world finally was ready for it.
If you consider anime filler canon, then indeed he is the original creator, but like another answer here points out, he didn't pass it on. Minato most definitely developed the Rasengan on his own. In that regard, they both are the creators, Minato just rediscovered it.
Nobody even knew who Indra and Ashura were before Hagoromo told his story.Indra are Ashura are ancient history.Not even Hashirama/Madara knew about them.
Yes,Ashura was the creator of Rasengan.In fact,he had already created an advanced version of it-multiple Rasengans.Howeved his jutsu was probably not passed down to the next generation,it was forgotten.
Minato re-discovered the Rasengan and essentially,became it's present creator.Rasengan is actually a ball of chakra-you could add a Wind Release to it and make it Wind Style and stronger as well.The reason why it's so powerful is because it's pure chakra,unlike other jutsus where you have to convert your chakra to the desired "style",like the Fire ball jutsu.
Clearly,Minato has no relations with the Otsutsuki clan.
You're already answered it yourself. It's a filler.
Aside from that, I don't think that counted as Rasengan, since it wasn't really a perfect compressed sphere, but a round unstable mass of wind release chakra. So you could say in this filler, Asura used a proto/pseudo/imperfect Wind Release: Rasengan. Also, the Rasengan was explicitly stated to be based off of the Tailed Beast Ball, which automatically makes it structurally and functionally different from whatever Wind Release technique Asura was using in this episode. It looks like a Rasengan, but it's not. Just like Chidori and Raikiri are similar to whatever technique the 3rd Raikage used.
There is no relation between Minato and the Ōtsutsuki Clan.
