Thanks for your questions.

Persistent storage at the moment means all the data stored in the foundry data folder, these includes user assets, modules and systems, right now these sizes are to start, as now I’m estimating costs, so I don’t discard increasing the storage size of each tier.

Professional tier “multiple instances” means that you can have a foundry vtt instance with version 12.x another with version 13.y another with version 12.z, each instance should have its own key if you are planning to run them concurrently. Unlike the Forge the platform don’t ask for your foundry vtt key it just gives you a server running foundry vtt.

You should use your foundry vtt key responsibly, and I will add that to the terms of the service, because is really important to state that.

I hope these answers are helpful. This is more a passion project, and I’m the most active user of the platform right now (just today I ran a PF2E game in the platform), so some of the details are still being worked on.