I'm not working closely with the Mac EC2 nor Open Source departments, so I may be wrong but the project should get credits covering a certain monthly bill, the amount would be requested on application for the credits.

It shouldn't matter what type of instances are launched and the credits cover any other services, not just the EC2 compute, as long as the bills are less than the monthly credits.

Use cases like this are far more than you can imagine. The longer I work for AWS the more I realize this...

what if the free credits cover 24x7 tenancy