[sento]

Happy or "seemingly" Happy endings are typical in Kinoko Nasu's complete works (Notes isn't really a full work, even Kinoko said it's more a mini-story about something bigger).

- In Tsukihime, 3 True Endings are happy (as Yoimachi Kanwa to Akiha True is the continuation). And Ciel True and Kohaku's one are happy enough. And it's said that he wrote first Good Endings, but then Takeuchi said him that the True Endings will be more fitting here (as Tsukihime it's Kinoko's darkest work, Fate is far brighter).
- Kara no Kyoukai = Absolute happy ending. And I don't see the "link" that you saw between UBW and Rakkyo aside from counter force (a thing that if you really are perceptive, is present in all Kinoko works in some way), because UBW is considered the route less Nasuverse related.
- Fate/stay night. Saber ending isn't really supposed to be absolutely sad, as even they accept it without any tears and looking forward for their own paths. Rin's True End is a happy one. And Sakura's True End too.

Fate/Zero is written by Urobuchi, a guy who didn't know how to write properly happy endings. Heck, he even wrote Fate/Zero with that in mind, calling Kinoko Nasu "the warrior of love". He said that he was looking for you, after the reading of Fate/Zero, run to your computer to find the happy ending that the characters deserve.

And the wall of text saying about the true ending. I think that the picture I posted clear it all. Really, in that dojo they didn't say "the grand finale, the Heaven's Feel True Ending", they say "the grand finale, Fate/stay night True Ending". And that answer, even if it's one answer, its the same answer that Kiritsugu found in Fate/Zero, even if he realized it late. So it's the only answer we had. You can say that it's the whole point about Fate saga. Of course, it's not the only ending possible, but they clearly said that it's the true ending to the whole story.