Actually, he and Balbanes weren't on opposite sides during the Fifty Years War. That war was between Ivalice and some other country (I think called Ordalia or something, I'm not sure). The war where you have Larg/Hokuten vs. Goltana/Nanten (the Lion War) takes place after the Fifty Years War ended. The Lion War was a civil war between different regions of Ivalice, competing for the throne. As you know, Balbanes died at the tail end of the Fifty Year war, a bit before the Lion War broke out.
1. If Orlandu were an enemy, I too would picture him as being a lot like good old Rubicant, from FF4. Being kind and respectable, even chivalrous, and then once the formal fighting begins, you'd be screwed. But I'd kill him. Just because I dont' lose in FFT anymore. I used to lose a lot, but I don't anymore. Ever.
2. I wouldn't use them at all. I never use monsters. I've never seriously made use of a monster. Except to ride Chocobos, and even then, I'd rather have another versatile human unit fill that space.
3. I've often wondered why they can't! It's not like a bow where you get the little two hand icon. It's considered a 1-handed weapon, and for some reason, you can't give them two!
4. Your word choice of "more useful" implies that they were useful, but could stand to be a bit more useful. I disagree. They were both entirely useless. I kept them only because they're unique. But quite frankly, when I got to a point in the game where I was recruiting a "Heaven Knight" and a "Hell Knight", I expected a little more than what was delivered. In fact, I expected A LOT more. I mean, those two couldn't hold up against anything. There is no situation in the game where they'd be useful. Even where their skill do hit the enemy, they really don't do that much damage.