Is there a way we could switch between Octane versions, maybe inside LW UI, without needing to have multiple installations of LW with different Octane version plugin in each?
Nope, sorry, this is not possible at all. The plugin only can be loaded once in each LightWave session, but perhaps you could use custom LW configs to don't need to install LightWave several times.
1. use two separate LW install verisons with their own self contained folders (put configs folder also inside LW folder not under ....user/.Newtek...) 2. Use same LW verison with separate 3rd party plugins folder and separate configs folders.