Advice setting up surround sound receiver

Apologies if this post is in the wrong forum.


I was using an old phenom desktop mobo connected to my tv for 3.1 channel sound. It's no longer practical, been using the TV's speakers more often than not. I got my 6y/o Yamaha RX-V681 out of storage.

I' never setup a receiver, been spoiled using a PC. I'm trying to figure out if it's worth trying any answers is appreciated.
1) It has 2 "sub pre-out" ports, for powered subwoofers. since manual doesn't mention it, I can't use my powered soundbar as a center channel, correct? ie AVR don't have a function for swapping subwoofer and center channels like sound cards have?
2)If I don't use a 7.2 setup, will it combine Rear/Side/Left Right automatically like a PC does or does the video need to have a 3.1/5.1 codec?
... the manual lists a bunch of 2.0, 2.1 configs (no 3.0, 3.1, etc) the next simplest is this 5.1 layout. I 6 inwall pyle speakers installed, they're terrible, I rather not use them I also don't want to buy a bunch of speakers for this.
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3) in regards to the in wall pyle speaker, I've read enough here that automotive speakers arent great for home theatre, but compared to pyle speakers... is it worth swapping them out.
4) If I can't use my powered soundbar, Is it a bad idea to use 3 old MCS speaker towers for Left/Right/Center?