I do not believe smell alone would be considered a breach of the warranty of habitability. In addition, if it was not noticeable upon inspection of the unit one must wonder how bad it actually is.
A wet spot on the ceiling itself is not a breach of the warranty of habitability.
The question is; what is the OP actually exposed to and does that breach the implied warranty of habitability? If it doesn't, there is no basis to terminate the lease. That is why I believe the health department or code enforcement must be involved. If they determine it is not habitable, then OP is good to go with remedies or termination of lease. It could be the simplest way out, unless landlord is willing to simply terminate the lease without liability on the OP.
I fully understand the OP's concerns but unless they call the government agencies, it doesnt sound like the landlord will (since it has apparently been like this for awhile so he most likely was aware of it) and if the OP just leaves they may have to fight a breach of lease suit.