Unless it’s been changed, the avatar getting out of the saddle is a function of cadence. When you’re climbing (not sure about on the flats, but I don’t think it does) and your cadence drops below I think 70 RPM your avatar should stand. I haven’t tried it in a while so it’s possible it’s been changed.
Regarding item 3, according to what the Zwift folks have said in the past that’s likely related to the rider your seeing having a really low frame rate on his/her end. It causes weird artifacts.
I also confirm that the 1/29 update created some error in the cadence on the CompuTrainer. The screen display seems somewhere between 5 to 15 revolutions higher. I confirm this by looking at the display on the CT head unit while riding the steady flat section.