I don't think this is actually true. Can you cite any sources referring to copyright as inalienable in the US?
However, "inalienable" was incorrect - you can most definitely sell your copyright.
Second, apparently I was wrong even then: see https://cr.yp.to/publicdomain.html
Face, meet egg.
if i read 17 U.S. Code § 201 (d) clearly, it suggests you can transfer ownership of copyright in any normal legal way of executing a contract.
I think your confusion is around the idea of works falling into the public domain via expiry of copyright, and/or works that do not have an explicit declaration of rights ownership.
In this case it seems she was pretty clear about her transfer of ownership. I don't believe you can rescind that just as you prefer, at any time.