Jetlang doesn't offer lightweight-threads. It uses an event-driven processing of messages (like Akka), so fibers can't block and you can't do selective receives.

The JVM's GC isn't per actor (I know actors are called processes in Erlang, but let's keep the nomenclature consistent), but its GC is extremely advanced, and some implementations work on a per-thread basis. The ramifications are that we can't offer the same level of isolation as Erlang, at least not on HotSpot. An actor could theoretically produce a particular kind of garbage that will cause a GC pause to the entire system. But other than isolation, the JVM is very performant (much more than Beam), and handles concurrency extremely well.

I don't understand your last question (on load distribution).