You're describing a VM with multiple languages. You might like the Racket programming language(s). It provides a powerful macro interface that can define new languages, and sandboxed execution to safely execute user code server side.
I'm pretty sure pedophiles will be looking for another platform following this news. Now any platform that doesn't scan for CSAM will be associated with it.
switch being underloaded to checkout is the same as checkout being overloaded to switch.
In the checkout model, branches are just named aliases for their current hash. This seems trivial to me. Do we really need to use another vocabulary spot up in our heads for a command that is strictly a more-restricted checkout?
https://racket-lang.org/