Lots of the considerations mentioned in your original post are room and preference specific. It would be pretty tough for anyone to give good advice without actually being there to help with the setup, in my honest opinion.
I would not use a car amp for a sub. It may work.
I'd be temped route your left and right signal through the sub if its an option.
I'm not sure I'd use an 18 gauge wire unless the receiver is very close to your towers. Even then I'd bump it up to a 14 gauge.
Just my $0.02.