First of all, I know Japanese (even though I usually read light novels just in English). However, I just play visual novels in Japanese (I can't bear hearing one thing and reading a mistake in the translation).
it seems it was only a suggestion or request to ufotable which hadn't been accepted by this studio
You're wrong there, it wasn't a suggestion, it was part of the way it was supposed to end.
This is Nasu's blog just after the episode was released. He says that the production of episode 25 started from a manuscript created by him (as the original author). Afterwards Ufotable had to trim/change things as it was too long/ too much of a novel.
This is the most important part that says that:
The manuscript he creates is 11 pages long (just text) and appears in Blu-ray box II booklet, which I happen to own.
It's not much but here's a summary (I can confirm it is good): http://kurozu501.tumblr.com/post/131376500337/kyuubi-hime-fateubw-bdbox-ii-booklet-scans
Proof that he didn't like the ending Ufotable made is that when it was released he basically said: This is what they did, and this is part of what I wrote as the actual ending.
Now, what do those 11 pages say about the ending?
Basically that Rin and Shirou will have their happily ever after. The part where Rin says that in the anime, that her priority is Shirou's happiness, that she will follow him, that they will suffer together but eventually they will get their happily ever after. All that was in Nasu's manuscript.
Nasu's saying that Rin will be with him (she doesn't abandon him) and that she knows that after some suffering together (as Rin says both suffer, Rin took part of that burden for him) Shirou will achieve his goal and they will live happily ever after.
Why do I know he doesn't become Archer? Because Nasu says so himself. At the end of the manuscript Rin puts her amulet in a box. She was touching it nervously and puts it there as a goodbye. Then Nasu says it means Shirou won't become Archer as he doesn't have the amulet. The sad goodbye symbolises her goodbye to the other Archer, the one that was her servant in the war.
The good ending is happier because Saber will also be there with them in their happy ending. I guess it is difficult to understand from the episode, but this is a literal translation of 2 paragraphs from his manuscript:
"The same library, a wide solemn space. I'm going searching for Shirou. On the walls and walls, heavy bookshelves stand side by side and contain huge numbers of books. Those books are all thick and thick, they are only periodical books. Shirou was alone in the reading seat. He is reading books eagerly. Image that seems to be studying judicial exam rather than studying magic. Shirou, on his way to self-study, raises his face from the desk and indulges in thought. The face of anguish, anguish, anguish is painful to watch. Rin watches it and calls out with the usual lightness. Dear "Thank you for your work, you were here after all." Shirou, I was a little surprised when I saw Rin. Shirou "Tohsaka? Not yet in class ......" Rin "I have finished the practice early" I Look at the book that Shiro was reading. A book of language, history, politics. Rin "Next to English is Italian? Magic to convey intentions"
"Even though you only have to master it, you are honest, is not it? "Shiro" Maybe, but I wonder if there is something that can not be conveyed unless I speak with the people of the country properly in the language of that country ". It is because Shirou 's honesty seems to be hunted down. Rin takes the textbook from Shirou and returns it to the bookshelf. Shirou protests with eyes only saying "What you are doing", Rin looks backwards and backwards, Rin "Freeze tomorrow, do not go out for a moment"
The next days Shirou waits for her in the lobby and they go to one of Saber's fake grave (yes, there are many of these as Rin says in the manuscript).
It's very funny Shirou has an "anguish, anguish, anguish" look while in thought and Rin comes to the conclusion: Ok, you need Saber..., I'm gonna take you there tomorrow...
The episode changes this as well. I'm not very happy with the episode actually.
Anyways, the answer is NO, he's not Archer. Nasu blatantly says so in the manuscript. In interviews he says the chance is so close to 0 it isn't worth mentioning. Here he just says he won't become Archer and before that he says Shirou and Rin will have their happily ever after.