A good local Realtor can help you with that.The advise that @Bjorn Ahlblad gave you above is categorically wrong and depending on state law, can land you in a world of hurt.For example, I can tell you with 100% certainty that in Massachusetts (I'm also licensed there), if there is a child 6 or younger living in the home, the owner MUST test for and remediate lead paint.  

Any ideas on remedy?

I have not done this specifically, but have had asbestos remediation training so am familiar with the issues. 

Once the seller understood that the remedy for us was for the seller to be forced to turn the property over under Specific Performance, but also cover all attorney’s fees and court costs, they begrudgingly moved forward.

So are 3 bed rental was apparently used as a meth lab and now requires remediation...which consists of complete cleaning and replacement of exhaust vents and cleaning furnace.

On the contracting/public-adjuster side we do mostly exterior storm-remediation work, and each year we have to break the bad news to at least a couple of our potential clients that their insurer is only going to cover say $4k of the $24k in work because they bought the cheaper ACV policy vs.

Does anybody have experience trying to remedy a broken contract anything similar to this?

However, many rent supply stores have them for rent and every insurance claim with odor issues (fires and water issues (mold)) includes there use in the remediation. 

Remediate the mold problem asap.

There are also remedies for that but it would probably be good for an experienced person to go through building and discuss options with these things in mind.