Today, mana is a play money system; however, would it be legal for Manifold to allow you to withdraw your mana to USD, so long as you were an accredited investor?

I suspect that this would be yes, as then your investments on Manifold would be akin to trading unregistered securities, acceptable for accredited investors. 

 thinks no, on the basis that our primary market type is a binary contract, which would fall under the purview of the CFTC.

Market resolves to whatever I believe to be the case at resolution time. If the question doesn't feel settled at resolution time, I will resolve to MKT with 90% chance, and ask a lawyer with 10% chance, based on a dice roll from eg 

(Nothing in this market or comments should be taken as legal advice)