Ubuntu Multiple MySQL instances and Apparmor
I ran into apparmor headaches getting multiple instances going on ubuntu myself.  Your configs look legit but I'm not super familiar w/ apparmor to spot a subtle problem if one lied there.  In my case I was just running multiple copies of the same binaries w/ different cnfs

Your initial error cited

name="/run/mysqld/mysqld-5-5-20.sock"


It seems maybe there's a config option that didn't come through in what you pasted that's looking to work with /run (instead of /var/run) ?