Nothing will change, that simply bypasses the HS200's relay.

There appears to be a command for it, I'll add it to AutomatoinManager for anyone that has it and wants to try. It'll be in the next beta version.

I don't know much about electrical regulations outside of NA so for all I know that might be normal, but it looks very dangerous to me. My guess would be TP Link is only using a single relay to pass...

Yes, you can chip out the drywall until the switch fits flush. It will be flush against the box in normal installations. Just be sure not to damage the drywall past where it's covered by the plate.

You need two black wires - line in and load out, and a white wire - neutral. If you don't have that 2nd black you've probably got a "power through fixture" circuit and the switch won't work there.

if by "replace the nonsmart switches" you mean change those to smart switches that will have no effect. If you replaced the old switch there's a small chance the new one is broken too, but more...