Mahayana is largely idealism,
If Theravada Abhidhamma denies the ultimate existence of external world (of chairs and trees, and other non-paramattha things) and merely calls them conceptual (which is a product of the concept producing deluded citta) then in a sense it is Idealism (or almost).
Personally, to me, idealism would really make Abhidhamma easier to accept. Less problems with how kamma & citta can produce all the results and all the incredible "worlds that one can be reborn into (to speak conventionally).
IMHO, a much bigger and fundamental difference between different Abhidhamma teachings is the nature of dhammas. Are they tri-temporal, with momentary manifestation? Or are they momentary only? In latter case it brings lots of potential logical problems and seems to violate the law of conservation of energy. IMHO.