Consider them all under copyright of his heirs. Copyright is really weird in terms of time limits, but given that he died within the past 40 years, his stuff will be under copyright protections for 70+ years after his death, plus even longer if his estate files for renewal.http://copyright.cornell.edu/resources/publicdomain.cfm
And if it turns out he didn't have children? (I'm not sure he was ever married either.)
Depends on his will (if any) or who was assigned to deal with his estate in probate. Usually "heirs" in a legal sense doesn't mean kids, but whomever he appointed to look over his literary estate--it could be his publishers, a trust, a foundation, etc.
Ok, I'll put in an inquiry with Night Shade Books.
You might want to also make inquiries with Centipede Press. They just did two gorgeous collections of his horror short stories, and I believe Stephen Jones (who was a good friend of Wagner's) was very involved in the project.
Good idea, John. Will do.