The term "isomorphic" led me to assumptions that weren't accurate and "universal" is so general as to be meaningless. Calling it "shared JS" is a step in the right direction, imo. I'm actually a fan of "agnostic" as a short form of "environment agnostic," but I don't care enough about any of this to try and encourage one or the other. I think the whole debate about this is pretty silly, lately (especially in the React world) terms have been shifting so rapidly that it's tough to stay on top of them without hanging out in the right chatrooms.
I totally agree with everything you said. However:
"There are only two hard things in Computer Science: cache invalidation and naming things."
-- Phil Karlton
How about "dual-ended JavaScript", sort of like "dual-ended mascara".
If we're gonna go that route, I still like "middle-end" JS better. :)