Startup menu tab

Where do I find it? If a choice on the 16.04 loading menu disables it, can the Startup GUI be started manually to reverse the choice of eg ‘auto sign in’?

Change the desktop?

Spent some time on the ‘how’ to change the desktop. All I’ve found is
frustration. Can you direct me to instructions on the ‘how to’, please?

Sorry, but I'm still not clear on what your trying to achieve. Add Mate to Ubuntu? For that just add the meta pack:

sudo apt install ubuntu-mate-core

This is now dated (14.04), but the packages are still in use.