I'm not sure I follow, but I think we're saying the same thing.
It's been too long since I've taken any hardcore discrete math for me to reliably reason about the bounds on the number of moves required to win. All I can do is make estimates based on simplifications.
How I arrived at my estimates for the order of magnitude of the minimum number of moves required to win:
At most, you can merge 4 tiles in one move. Assuming you were doing 4 tiles every move, and the game just didn't produce twos, it'd take just 128 moves. Order of magnitude: 10^2.
Assuming exactly one merge per move, and that the game only produces twos, it'd take 1024 moves. Order of magnitude: 10^3.