Can the REPL on the server be used to update production systems with no downtime?

For distributed computing across multiple machines, you are right that the actor model has an edge there. Most applications don't require that, however. There is however Onyx and other tools for this in Clojure:

http://www.onyxplatform.org/index.html

I think the choice between Clojure and Elixir really comes down to how much your particular idea or requirement genuinely depends on the actor model. The vast majority of applications don't specifically require that, in which case you have more flexibility.

The vast majority of apps don't require distribution but I'd like to use a model of computation that would support it when needed so as to avoid large refactoring. And, preferably, allow for code reuse with systems that are distributed.

EDIT: Watching a talk about the internals of Onyx made by the creator. I gotta say, I'm liking it more and more.

https://github.com/gaverhae/okku

(This is just one way to use Akka).

You might just have converted me to clojure, haha.

I think I'll start with a hobby-project and see how well it goes.