pgp on macOS 10.12, Sierra Rrunning macOS 10.12, Sierra, on my secondary Mac and am fairly happy with stability and all that.

Nonetheless the one thing bugging me to the max is the incompatibility of GPGTools. I know that it isn't GPGTools fault. What would be a recommended alternative to decrypt gpg/pgp emails coming into Mail 10.0 ?

Yes, I am aware that I shouldn´t be running production stuff on a beta.
I'm experimenting with veertu to run a 10.11 VM inside 10.12 native for cases like this. It's clearly not ideal, but depending on how large your mailboxes are - it might let you keep working with encrypted emails while that specific tool gets reworked (assuming it does get ported).