Providing house wall switches with 3.3v/5V supply?
My opinions:


Don't trust a cheap Chinese design.  They are sometimes lethal.  Copy a good design from a respectable source
Depending on your country there may well be regulations preventing you from running low and high voltage cables together.
Nah, you'd be changing them every couple of days.


What I would do:  Just buy the plug-in remote switches - they're 10 a penny these days.