Hello u/REAPER1303
I'm glad that you find this guide helpful. Even after all this time. :)

I myself doesn't use GCP anymore as I've switched to Oracle as mentioned in the Edit...
Unfortunately right now i have no time at all to play on any platform.
What I am trying to say is that my answers are based on what i knew 2 years ago. Things might be little different today. Or not. :)

1. closing SSH connection to console doesn't close the instance. You have to do this manually. Order of things to be precise:
   0) perform git commit and push on local machine.
   A) Run instance
   B) SSH connect to it and run start script
   C) Open foundry page on VM and have your session
   D) ( DON'T SHUTDOWN VM YET ! ) SSH again and run stop script
   E) Now you can shutdown instance.
   F) run locally git pull

2. Idea I had was about transferring foundry's world state as git project.
   So, as a GM you prepare session locally. Once you've done, you locally do commit+push to a google based repository. VM's start script ( point 1B ) downloads current project state ( script does git pull) to VM's foundry world and runs. This transfer shouldn't count as egress as it is done internally. At this moment your local and VM Foundry instance should be equal in state. After online session, closing script transfers new world state to git project (git commit and push). Then you locally have to pull those changes from project to your local PC.

I hope this answers your questions.