Any way to hide particular actions without deleting them?
Not possible. Of course, you can collapse a folder group of Actions, but you already knew that.
It doesn't make any sense, I use the Button mode. It's a pity.
As there is no hide option, there are only hacks. Is this for any "random" loaded Action, or only for specific "predefined" Actions?
I don't know what you mean. As an example, we can create 2 FS actions - for 8 & 16 bit, plus a conditional which chooses one of them, depending on the bit depth. So, the two main actions could be hidden.
I was thinking of a hack for sets, not actions in sets, now that you have described a scenario, it wouldn't work.
The only alternative that I can think of is using colour coding and naming to make the required "run" action obvious and stand out from the "supporting" actions.