Language intricacies aside, is there a reason to use Clojure over Elixir, Erlang? Genuinely curious what JVM has to offer vs BEAM / OTP if you're going to use dynamic languages.
Sadly no, but I don't think there's much of a ping difference on a good network. With WebRTC (datachannels in UDP mode) you just get the benefits of UDP (or lack thereof :)).
Only played it with my friends, don't think a map ever exceeded 10 players. So I can't say much about scaling - although it was a client-server model over UDP and not peer to peer networking.
Quake's source code is very hackable. I forked ioquake3 for a personal project and it was surprisingly easy to add WebRTC support to the C codebase, so you could play it in your browser over WebRTC :)