Strategies outside of legal remedies are called "Self help" measures and, at least in my state, are frowned upon by judges.

Landlord shall use reasonable efforts to remedy the plumbing problem.

If you mean suing your tenants, that wouldn't work because the court would say you have a statutory remedy already - you have the right to their security deposit to use for rent, after they vacate - and you have the eviction remedy - normally.But, the reason you can't evict right now, is because the courts are closed. 

Remediation will likely require testing and removal of a substantial amount of soil if the spill covered more than a tiny area. 

Also we had spent a lot of money remediating a mold issue a year before.

Also, the as-is section in common contracts here states that the buyer's remedy for an issue found in an inspection is to cancel the contract and they are not to provide the seller with the inspection report.Your contract should still include an inspection contingency which will give you X # of days to inspect, and then cancel the contract and receive your emd back if the condition isn't acceptable to you. 

Without professional testing and without permission to enter the property, there is no way to remediate the problem (if there even is one). 

You just need to find out why it was condemned and how to remedy.

I'd come back with the issue and see if they will remedy..

It sounds like your state may have more stringent requirements as regards remediation in those situations where you do have knowledge that lead is present...