I'm not creating the false dichotomy, merely reacting to it in others. Garak there "can't imagine" any ruler not playing the 'cynic' (in game terms), using a religious excuse when convenient and ignoring religion when it isn't. I'm pointing out that there actually were people (including rulers) who took their religion far more seriously than that (without going to the 'zealous' extreme of wanting to exterminate all infidels & heretics). Marinaliteyears agrees, and thinks it likely for people to change religion like clothes.
Isn't Marinaliteyears basically arguing the same thing as Garak? Basically the whole cynical opportunist type thing? If anything thats more 'free for all' than I'm arguing for, I just think that a blanket ban on intereligious marriage was a really heavy handed way of dealing with the issue.
All that said, I somewhat agree with his point, albeit from the other angle. An actual zealot will likely act as such, but a cynical ruler may well still use zealotry as an excuse when it suits him, and throw it aside when it doesn't. He might not like heathens as such, but he's likely willing to negotiate with them. In game, you cannot create impromptu alliances; marriage is the only way to do so, and as such, the complete inability to marry heathens no matter the circumstance significantly decreases depth in diplomacy at borders between religions.
As for your courtiers, while they're technically not slaves, they come damn close. There are a lot of decisions you get to make for them without their input. When you marry in your court to out of court, you and the other ruler have to negotiate.
I was more suggesting that people shouldn't really be OK with the king marrying a lowborn heathen rather than the lowborn heathen getting a say in the matter.
I haven't dealt with the sympathy traits yet, but they seem rare enough not to help what you want much.
My point wasn't specifically about the sympathy traits. It was that the likelyhood of accepting heathens marriage offers could easily be affected by traits and situation. Presumably it would automatically be difficult to get such a marriage (and impossible unless you were reasonable close to them). Cynical would perhaps slightly offset any hesitance for marrying a heathen, the sympathy traits would eliminate it while conversely, zealous would make it almost impossible. The game could do with more depth from both a character perspective and a strategic one and using traits like this more would provide that.
Regarding "best of both worlds", if it's doable, it's easily gamed by players to make such limits fairly academic.
Everything gameable will be gamed by the player. That said, there is nothing gameable that this will introduce thats not already in the game, and nothing that will harm gameplay. Being able to marry heathens is possible. Raising your child with a heathen to convert is easy. 'Eugenics' won't really increase dramatically just because you have a few more people to attempt to breed super geniuses with. The only thing it would allow you to do that you cannot now, is gain an alliance, which is more realistic than any of the things you can currently use marriages with heathens for.