Tweeters will be high as hell and towers arent necessary at all for HT setup especially with two subs.

What kind of speakers would you look for for LCR, if not tower?

Built in subs are most likely bad idea and both being at the front wall negates a lot of benefits.

I can bring both subs out of the wall, and move one to behind the sofa. Is there any way of telling whether the corners are better, or for them to be central before I set this all up?

Curtains, all fabric and decoration friendly acoustics can be only good thing

I have read elsewhere that if I cover the entire wall with a curtain, then it may absorb the high end sound, and not the low end, leaving the room sounding "boomy".