to combine groups, see image. (this appears when you right click in the groups area of endnote on a PC anyway).
what you describe in your images, is pretty much how I add keywords to a group of references. Then I let the smart group settings add them to the group.
I also create a “hard group” that isn’t smart to “freeze” them, as smart groups aren’t synced (updated Nov 2020, I do think you can now sync smart groups, optionally). I drag and drop the whole group when the numbers don’t match, from the smart group to the regular group of the same name (although sometimes I include (smart) as part of smart group names – see image for example of that too).
