I play minecraft with the break/place buttons reversed from default, but I play FPS games with the default aim (RMB) and shoot (LMB). Is there a way to rebind the controls only for CGM and not all of minecraft? Cant find it in the configs or in game settings if its there.

I'll look into it when I update the mod to 1.19.4

is it possible to be added to 1.19.3?

Added in 5acf008

Unfortunately due to technical reasons, you won't be able to use offhand items like a shield. This is not something that can be fixed, the gun mod is already performing lots of hacks to get the current control system working.