Can't answer all you questions but I do have a 2012 3720 with autoconnect deck. If you are looking at a deck it should be easy to tell if it is autoconnect as the PTO shaft stays on the tractor so it will not be on the deck when it is off of the tractor.
As for the brackets, I don't know for sure but I do not see why the autoconnect deck wouldn't fit on all the 3mx20 model years unless there were changes made to the major structural components of the tractor, which I doubt happened.
I can tell you that it works great and I would hate to go back to the roll under deck that was on my previous tractor.
Hope someone else can shed more light on this for you.