And I have no problem with the JVM interop. As long as you don't try to write Scala libraries that need to be consumed by Java applications, everything just works really.
Scala on a well-tuned JVM can reach much lower latency than you might think. Yes, in theory you'll hit a limit that Rust could help you go beyond. In practice, are you ever going to reach that limit?
Rust's embedded support is mostly theoretical at this point, and the use cases for old-school embedded are increasingly limited. Want to write some code for that cheap off-the-shelf SoC you bought? Scala is probably a perfectly good option, because it's probably an ARM with at least 1Gb of RAM and the JVM will run there just fine. If you're actually writing something that has to run for months off a single charge then yeah, Scala won't cut it whereas Rust may one day be able to. But most "embedded" these days isn't like that.
No, it's not. You can call native (C) functions from the JVM.
That said; nowadays you have the Native and JS runtimes as possible targets as well. Scala.js works surprisingly good with very few caveats.
However, I must say that Rust is also a very productive and quite powerful language (albeit IMHO not as powerful as Scala) and Scala is actually not that bad at performance either - probably better than a vast majority of other languages out there (albeit not as powerful as Rust or C++).
As far as ecosystems are considered - this is really hard to say. Rust has the whole C (and most of C++) ecosystem at hand plus a few really amazing Rust solutions like Cargo. Scala is limited mostly to JVM and JS, which are great ecosystems, but it is not true they offer everything. And there are still some caveats when using Java tools with Scala (e.g. profilers, debuggers or build systems).