What you should probably do is to tidy that up so that you can use it. It's photo time...
Yes/no, see below.
You'd have to - the permanent live comes from the lighting circuit, if you replace it with one from a socket circuit the lights would have L&N from different circuits, and that's a no-no. And will trip the RCD for the sockets, if there is one.
Yes, and not really.
Think about it. Imagine you'd just moved in, and the previous occupant had done what you suggest. You decide to replace the light switch, so you turn off the power for the lighting circuit, unscrew the switch, and oh dear, there are still live wires in there because they aren't on the lighting circuit. This would be a Bad Thing™.
You can do it if you can label the switch to warn of the need to isolate more than one circuit in a way that the warning is seen before any access to potentially live parts is gained, but that's not really possible in a domestic environment unless you use a grid switch. Or you don't mind the warning notice on display.
Better, as I said, to sort out your spaghetti junction and put the wall lights on the lighting circuit, so lets see what it likes and maybe some suggestions can be made.
