Iran - International | International treaties adherence
you aren't party to a treaty unless you agree to be a party to it.
you're right tat they have violated the npt article 3 by failing to declare their enrichment in 2003, they agreed to the npt in 1968 (pre revolution). we (well, the american gov) believes they broke article 4 (but the proof is circumstantial).
there's no guarantees this works, but i do believe it's worth a shot, if it fails, the sanctions come back with full international support (if we kill the deal now, only partial sanctions would remain in all likelyhood), and if we fish for a "better deal" we likely get no deal (and again, only partial sanctions).