I agree.

My Rule of thumb  --

remove the builtin/administartors but remember to make the DBA groups sysadmins before that

Just a guess but couple of things worth checking :

check if that instance is listning on multiple ports.

Serach the registry at the client to see if there is any entry...

I am wondering what are you looking for in the PSS (process Status structure).

This is an internal command used by Microsoft support engineers which I believe would not be of use...

I would recommed doing an In place upgrade for the instances . but make sure that you have backups of the existing databases before you start anything (and  a lots...