First, the plastic should peel off. It may leave a bit of a adhesive residue which is easy to take off with acetone and probably lacquer thinner and lots of other stuff. I wouldn't try ammonia; not that I know anything about it but the petroleum based solvents won't hurt the aluminum; ammonia might.
I don't think it is unreasonable to ask to peel off some of the plastic. Van's advises that it be removed ASAP, so no harm should come from removing it. Besides, it gives you a chance to check for corrosion under the plastic; it does happen especially near edges. Holes are edges, too, so inspect around holes as well as edges.
If you buy the project, the easiest way to remove large sheets of plastic is to peel off 2 or 3 inches, tape that to a broom stick or similar object and using vice grips for a handle on the broom stick, roll the plastic up on the broom stick.
1941 Interstate Cadet