Yes you can easily REPL into a live remote server and work with it while it is running. But more typically, the REPL-driven development happens on your own machine while you build, and then you just deploy the whole app. But it can be useful for remote debugging when necessary.
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:
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.
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.