massa will be back in f1?
We Won't know until the deal with Bottas is confirmed first and then waiting until some point in January for Williams to confirm their own line up. But Claire Williams specifically stated that Bottas won't be released until someone as good as Bottas is can carry the team forward and perhaps even help Lance Stroll out as he develops. As you mentioned their only focus is potentially Massa as a solid 1st driver to carry the teams hopes.
If the deal does go on, Williams will make a large saving on Mercedes engine costs in exchange for Bottas.