I liked Yusuke and Keiko's relationship for what it was because the writing was very tight anime wise(and the fact that they already had a relationship that bordered on love before the story started also helped)...but i'm one of those people who generally don't take to relationships that clearly supplant one or the other like other takes on that type of relationship tend to be in anime or manga.
Keiko herself tended to be more of a non entity more than anything in the series despite being properly portrayed as Yusuke's human chain, and characters just being defined by their love interests have the potential to go wrong if the characters and writing arent strong enough to hold it up, which certain mainstream shounen show to a very disparaging degree (inoue, hinata ect). I've even had my criticisms of Sakura to some extent with that, although i respect HF infinitely more as a story than i do a lot of others i could name with similar characters archetypes, some of which i've already mentioned.
As for Arturia and Shirou, i think your draws against Shirou and Arturia are fair, i've heard them before and i've respected those views. But I feel like it really fits the fantastical fantasy "soul mate" ish setting the story is going for, and the development in the story from both characters perspectives works on a powerful level even if i feel like there are many improvements that could have been made to make it much better than i thought it was.
Even if the two week time frame does indeed feel too fast, i feel like the same criticism can technically be applied to all the relationships in the story in that sense, and Shirou's character development over that time. I don't think it would have been too hard for Nasu to stretch out the grail war for a few months at least, the same events could technically take place over such a time and be more realistically spaced out.
But i guess when your fighting a magical death war in which you have to make snap choices, depend on people and really come face to face with the reality of a situation to survive, any major changes to your outlook on life and the people around you can be excused to a certain degree.
Not really, just due to the structure of how the VN ended up and what priorities were emphasized in the route itself. Honestly considering that, Nasu did the best he could under those circumstances i feel.
The route is about, for the most part, Archer and Shirou's ideals and coming face to face with what Shirou does peering into the end result, and all the other characters pretty much are intended to take a backseat to it being Shirou's(or Archer more specifically) route more than anything. For it to truly be Rin's route, i feel like a completely different story would have been necessary, but then it would not be UBW.
FSN itself is shared between Rin and Shirou in prominence of screentime...so technically saying UBW is Shirou's route is somewhat misleading since all the routes are Shirou's that he shares with other people(Shirou/Arturia, Shirou/Archer, Shirou/Sakura/Illya ect)