Brendon Urie, Elvis Costello and others doing backing vocals of previous Fall Out Boy songs on "What A Catch, Donnie"

Sam Carter from Architects on "Don't Wait" by Neck Deep

Corey Taylor, Steve Vai, Dave Ellefson, Neal Morse and others guest spoken words on "Repentance" by Dream Theater

Liza Minelli as the titular "Mama" by My Chemical Romance.

And OP, I'm pretty sure it's Synyster Gates that does the guiar solo on The River too.