Rust doesn't "depend" on LLVM in the sense you seem to imagine, you can instead lower Rust's MIR into Cranelift (which is written in Rust) if you want for example.

LLVM's optimiser is more powerful, and it handles unwinding, so today most people want LLVM but actually I think LLVM's future might involve more Rust.

It can, but until that becomes the reference implementation backend, it hardly matters.

Similar to how much Python folks disregard PyPy's existence.

I doubt LLVM project would start accepting polyglot contributions, beyond what they already do for language specific frontends.

Also, the ongoing GCC support is dependent on C++ as well.