As far as I remember mrustc is not a full rust compiler/frontend, I.e. it is only usable with rust code which had be "proven" to be valid/correct rust by another compiler so you can't really use it for development. It's still useful for bootstrapping rustc as it's a rust compiler not written in rust. It probably can also be useful to compiler rust for some exotic hardware architecture which had no llvm support.

That's correct. Moreover, it's only attempting to compile rustc. Which means it could be completely broken for some other existing rust code. It is not recommended to use it for anything else than bootstrapping rustc (but this is already an incredible achievement).