One large fan isn't possible because there is no space because of the PSUUgh, no. Go for a single large fan. I had double 80mm in the back of my old case and replacing those with a single 120 was the best decision I ever made before going SFF
And I'll filter all the intake fans, those at the back are exhaust fans.I think you should try and go for intake fans only that you can filter, it's easier to keep the dust out of the case that way.
If you have a 5.1 set connected and want to use headphones you can switch between the 2 more easily, other then that I also don't see a reason for more audio ports if it's on the back.
Also the one true number
Graham's number
Yeah I need to see that to understand it properly