AoA does have its own buses. The lines are usually fine as buses are maybe 5 minutes in between each other for each park (10:00 bus leaves to MK, 10:05 new bus for MK arrives) was our experience.
LM is not as far as many make it seem. Triton building is the closest, then Ursula, farthest Ariel. 5 minute walk if you walk slow, 2 minute if youre fast.
Coming back from the parks, buses are usually lining up right after each other (once again, our experience). If you are lucky enough to have a POP bus filling up with a short line and an AoA bus is no where in sight, take the POP bus. The walk from POP to LM is worth it instead of waiting around for an AoA bus. Maybe a 10 minute walk at most (as I said, depends on your speed) back to LM.