As for why Solas could be race gated...at this point, its anyone's guess. It could be they just thought Fen'Harel would never find interest in a non-elf. It could be that future plans involve rebuilding an empire, and a non-elf would not make sense to be intimately apart of that. His actions could restore elven immortality. Or, it could simply be that they wanted to write the romance with a heavy focus on elven lore and customs without taking the time and effort to stop and explain them for non-elves.
Let me distort what you wrote there
-Waves an extravagant, over the top team optimism flag-
How about!
Whatever Solas does returns immortality to elves, and since Solas is afraid of dying alone (and perhaps watching loved one dies) then Solas will feel comfortable in the relationship if the lover can be immortal as well? Humans, dwarves and qunari won't fit this position, Solas would've had to dump them.
That way Solas & Inqi can be together forever? 
To be honest, I just like to think that Solas just has his preferences.
I got a friend who likes pale skin and dark hair (he really isn't attracted to blonde hair), nobody questions him, they just accept his preferences.
I have a friend who is really put off by skinny, I got a friend who is put off by even slight chubbiness, I got several friends with redhead fetish (it is a beautiful hair color, though it makes me think of Iron Bull >_>, Lavellan is a redhead, I felt disturbed for her when he went 'mmmm... redheads'), I got a friend who pretty much refuses dating guys who are shorter than her (and she is quite a tall girl, so they are hard to come by).
They are all good people, even if I'll never understand them ^^;
Heck, when I was younger I had my preferences as well - they were entirely inspired by Johnny Depp (now I basically know that I'm attracted to intelligence, if a guy is smart [plus nice, obviously], I can find him handsome, even if I thought otherwise before XD [coughSolascough]), I couldn't imagine myself dating a white nerdy guy (even if they are usually spot on personality-wise).
So, basically what I'm saying: whatever the reason for the gating, I personally feel it makes him more of a real person, even if the gating is inconvenient. I don't think they need to justify the gating, like they don't need to justify Dorian being gay (a lot of people wanted him to be romanceable for females, he is, objectively, really handsome + awesome personality) or Cass being straight, or Vivienne/Varric not even being options.
The only thing I'm curious about is the "super story" thing, I just hope it means DLC.