ZFR: When you reach gates to another world, you get a message that you can pass through if you got MM2 installed. Was it actually possible in the original Apple/NES game to move characters to MM2 this way?
It is definitely not possible on the NES version, as the game was saved to the cartridge, and NES cartridges are not hot swappable, not to mention that MM2 didn't actually see an NES release.
Note that, when transferring characters, stat and resistance bonuses from equipment become permanent, subject to the limits imposed on transferring characters. (Note that, for some races, the race's natural resistance doesn't even transfer fully, so new characters might have higher resistance than imported ones; the only way to permanently raise resistance in MM2 involves what could be considered an exploit.)
Also, the NES version has one flaw that can be very frustrating: Locust Plagues are ridiculously powerful in that game, being able to dealt 255 damage (= instant death) with their attack. To make matters worse, when this happens, the round would restart, and since Locust Plagues are really fast, they would probably kill another character before you get a chance to act. Repeat until entire party wiped out. Do you think this is fair?