1. It sounds like it, but it's a political marriage, to increase your support from one of the factions. It's not a marriage for love. That's why you can't marry a normal human.
You can take their story in a non-romantic path if you choose (or a romantic one, probably.) Each princess has a story arc that has branches and different ending points. I bet the relationship also affects and is affected by your relationship with their race.
3: I think they said that there are at least 13 types of ground units. There may be as many as 15 types, I'm not sure.
