Hi John,
I'll do my best to answer...
1. I'm not aware of a keyin that can flip the hand direction during placement. The best I can think of is that after selecting the location within a wall but before accepting , if you drag left-right for horizontal walls or up-down for vertical walls that should flip the direction.
2. I believe most/all of the content is created by regional folks, both within and outside of Bentley. So I can't really say why sliding doors are not part of the NM dataset (I believe a few other datasets do include them).
3. I do see four overhead doors in the NM dataset:
