Regarding C - yes, multiple stacks should be supported, and I literally opened a PR to add coroutine support based on JSPI to emscripten: https://github.com/emscripten-core/emscripten/pull/25111

JSPI-based coroutines are much faster than the old Asyncify ones (my demo shows that).

As for your core message - I'm just the user, but if Google engineers were able to implement that, then it is possible to implement that securely. I remember Google engineers arguing with Apple engineers in GH issues, but I'm not on that level, I just see that JSPI is already implemented in Chrome, so you can't tell me it's not possible.