If your Jeep has the Carter 1BBL does that mean that your Intake can only accept 1BBL Carbs?
Yes. Unless you get an adapter to put a 2bbl carb on your 1bbl intake.
Is it possible that the Jeep has a 2BBL Intake with a Carter 1BBL? If yes, how common is this, and what is the easiest way to know?
Possible, but highly unlikely. It wouldn't be a factory setup, but some previous owner might have done this for god know's what reason.
I would like to upgrade my Carter 1BBL to ether the MC2100 or a Weber. If I have a 1BBL Intake, I know that I can ether get a new Intake, or an adapter plate. Yes.
I know the plate would be cheaper route. What are the advantages (besides cost obviously) / disadvantages to the adapter plate?
Advantages: Cost. Can't think of any others.
Disadvantages: Not getting full performance potential from your new 2bbl carb by restricting it via a 1bbl manifold. Essentially this is like a NASCAR restricter plate. The new 2bbl will still give you better performance than your old YF, but not as much as you could potentially get by using a proper 2bbl manifold. Or even an aftermarket 4bbl manifold with a 2bbl adapter.