Hi rasto, welcome to the forum!
I don't have a 912, only a 902, so I can only answer for the features that are the same in both devices. But I'm sure some helpful owners of a 912 will drop in soon.
2) yes, often it is even better than portrait mode
3) jein. They are not open side by side, but you can switch between them quickly.
For 1) - there are already many threads about annotating here, maybe they will answer your questions.