Quickly iterating on UI?

Is there a way to quickly re-launch the UI application without having to completely restart LinuxCNC? Can I just launch my (from-scratch) UI executable and it will just "find" the running LinuxCNC?

Hard to say without code to look at.
but yes it's possible to run/reload a separate ui. That is what you are doing when you load a gladevcp panel.

One way is to load say AXIS and the load your screen with a HAL command.
loadusr python3 myScreen.py

In this way AXIS can run the system as you develop the new screen.