Probably never. Gnome is working on their own mobile port. If I'm not mistaken this is partly due to the fact that Phosh and Gnome diverged too far from one another. Although I think some Phosh code is being reused.

Gnome Shell and Mutter are based on completely different technology(GTK instead of Clutter) and compositors(wlroots instead of Mutter). There is nothing that can be merged from one to the other.

Depends, guessing from this conversation I'd assume there's currently nothing to thank for.

And because of that: If you like Phosh, consider giving back:

• https://liberapay.com/Phosh/ and
• https://honk.sigxcpu.org/piki/donations/ are simple ways to do so :-)