Let's go with something simple...

A Tia and a female genuinely falling in love would most likely lead to them leaving the tribe. They may try to start their own tribe (easier said than done) or simply emigrate to a city-state and live otherwise normal lives as citizens therein. In the insular tribes we've seen in-game, though, I doubt it's a frequent issue - their societies frown on mating with a non-Nunh (and you'd be surprised what that kind of pressure will make people do) and there are few (if any) Tias hanging around their settlements.

As for G'raha and his magic eye thing... well, I figured maybe it was a recessive trait and only showed up under special circumstances. Given it's related to Allag it's also very plausible they programmed it to show up in the first child sired by / born of those who carry the gene, so there will always be someone capable of messing with the Crystal Tower until it's dealt with. G'raha simply wasn't strong enough to become Nunh... or didn't care, I guess.

I'll leave the whole Hyur / Mi'qote thing be. Best not to think too hard about it...