1. The walls never seem to be in the right place for in-wall, unless the room was designed for audio, so not that. THe only exception would be s second system (bedroom, etc.) where the listening isn't as critical.
2. For my main system, I've always had front towers, and 5 or 5.1 surrounds if this is a place to watch movies, so this is a good option.
3. Soundbars are good for what they are, but not good enough for me, unless this is a kids room, a place you don't wnat to run wires, etc. So no in most cases here.
4. I'm not a fan of on-wall, so no. I'd go in-wall if you absolutely must hide things.
5. I've never tried wireless speakers. I'd be worried about sound quality ans signal quality, nso no, not now.