copyright law enables the original author or heirs to terminate agreements after 35 years -- you can stop looking. There is no way to stop the process unless you can somehow turn back the clock. The only good news is that you can continue to exploit derivative works created prior to the reversion ... in other words, you can continue to sell and distribute authorized versions or adaptations. Note, also, under the law no new derivatives can be created (unless authorized by the reversionary owner). This Author's Guild article explains the reversion rules.
California work made for hire question ... We're familiar with the rule that works made for hire agreements in California may create an employer-employee relationship. (We've written an entry on the subject.) But -- after several passes -- we're still not understanding your question. If you're concerned that work for hire contributions will create an employer-employee relationship have the donor assign copyright instead of using a work made for hire arrangement .