To my non-legal mind, the strategy of making a crime legal after the fact, sounds a lot like ex post facto law, which Wikipedia assures me is not permitted under Article I section 9 of the constitution.
So how are they going to get around that detail?
An Ex Post Facto law is one that criminalizes an action after it occurs.
What is being discussed is decriminalizing actions after they occur, which is Constitutionally acceptable. In fact, the Constitution expressly gives the President the power to pardon, by which he could effectively acheive the same end by granting a blanket pardon.