"Out of interest which compositor are you using?"
Compiz 0.9.12.2.

One more issue I forgot to mention, 5: while using HDMI monitor and pressing fn
buttons to switch screens it sometimes gets buggy - the monitor plugged into
hdmi divides itself every time when it happens, so I see main screen and it's
smaller copy in left top corner, also screen doesn't react.
Might be this the case of adding custom resolution to monitor in
10-monitor.conf? Monitor was missing at the begining 2k resolution, I've added
it manually (monitor supports up to 4k res).

Back about 4th issue: this mostly happens while running Steam, Blender and Atom
text editor or Chrome. More native apps like Nautilus, gedit work great using
DRI_PRIME=1.

These all issues can be annoying sometimes, especially freezing while closing
fullscreen, that's the killer one.