After the legalization of marijuana leads to a dishonest pot-dealer receiving a huge tax bill, the de-motivated must become motivated before he can lead a one man crusade against a clean-cut district attorney to have the drug 're-criminalized'. (Am VERY interested in hearing some alternate titles for this. The punnier the better!)

Others have misconstrued what I was aiming for but I think you get it Nic, yeah? Of course there are all kinds of outrageous concepts in film and tv, but every world you build has to have its own rules to make it believable within the context of that world. I'm not saying every story has to be rooted in real world facts as we know them... good grief (eyes rolling)

I think that with this premise it is not about making it believable rather plausible. Pineapple Express was far from believable but was perfectly plausible and its that connection to plausibility that made it's naturalistic performances funny and the premise fun.

Otherwise the story goes into farce territory, which is fine, but from the sounds of the logline this was not the intention.
However if farce is the idea, then any structure will do really. Chich and Chong building a car out of hemp to smuggle it into the US from Mexico was pretty much a farce that found it's own audience many years ago this could perhaps work now days as well.

I for one prefer a comedy that is hilarious because it is in touch with reality. The beauty of well crafted comedy is that it provides grounds to critique real life and allows us to say things that we other wise can't.

Forget believable, engage the audience.
Your movie could be a mix of "Pineapple Express" and "The Campaign".
What if your pot head is selected by the special interests as the front man of their campaign. He may genuinely want to re-criminalize drug, and with their help he may suddenly succeed.
You just need to find a reason why they would want him as the front man.
May he is an accidental hero, he saved the life of a kid on live TV and the kid made him reject drugs.
Maybe he was caught swearing against drugs on a YouTube video, how they have ruined his life. Due to a mix up, People thought he was against the legalisation of drugs as a question of principle, when in fact he has just received his tax bill and was thinking how legalisation had ruined him and was taking the fun out of it.