3, 4, and 5 you all get explicitly by the rules (along with no checked bag fee). 1, as I noted, is probably irrelevant since I don’t think there are many airports with priority security anyway. For 2, you can probably show your connecting F boarding pass and board with group 4 or 5. Policy certainly doesn’t allow that, but my personal ethics declare it reasonable.

You asked about an agent manually changing the group on your boarding pass. Is that even possible? I doubt it. Pretty sure the group is determined entirely by the computer based on fare class for that segment and status of the passenger. If you’re booked in T (economy award) with no status, the computer will give you group 6-8 or whatever it is and there’s nothing anyone can do about it short of rebooking you in a booking class that gives you a different group. If you really care, ask the gate agent if they mind if you board with group 5 if you don’t clear into first. But why does it matter? Since you’re checking bags, presumably you’re not fighting for overhead bin space?

Of course, this situation (booked in coach to connect to long haul J) happens all the time for people starting on a one-class regional plane. Same rules apply then. One of the advantages of making domestic Main Cabin Extra a separate premium economy booking class rather than a paid seat assignment within coach is that it enables giving MCE to people in your situation. AA hasn’t done that (a choice with significant other benefits for customers), which makes it so that you are not entitled to MCE in your situation, according to the rules. Of course, can’t hurt to ask and hope for a sympathetic agent; I agree that by principle AA ought to put you in an available MCE seat.