Your PC is shutdown if u press start and shutdown. Default for power button is sleep!
1 USB on right side, if u re-plug will still get power through AC if PC is shutdown, so unplug AC.
Steam button, as far as I know acts as a secondary power button but also launches Steam as soon u hit desktop. Avoid it like power button if u clean keyboard.
It's called fast startup and I don't think it affects anything. As far as I know Instead of closing everything, fast startup saves some system stuff to hard drive so it wont have to start from scratch and can startup faster, it will still shut your PC down.
If u open up your laptop, unplug AC(of course) but you also have to unplug battery!