Why do you think Q was so upset with his son for messing with the Borg, could the Borg possibly pose some sort of thread to the Q?
I don't think the Borg are any type of threat to th Q. Instead, I took this to just be an exasperated parent. Essentially saying ... "Don't do this, because I don't feel like cleaning up after you... AGAIN!"
Was he worried about Voyager since we know he's more then capable of growing attached to a group of mortals despite putting up pretenses to the contrary?
I doubt it. I think the Q see various races as like pets. He's fond of the "puppies" on Voyager, but for the Q, nothing can be done that can't be undone. If Junior kills them, he can just bring them back. In keeping with my first comment, he's just tired of doing it.
Do you think maybe the Continuum who fancy themselves the "Guardians of the Universe" find that interference with the Borg can have galactic repercussions, and so it's highly frowned upon? It was in fact after 'Q Who' that the Continuum made Q himself mortal, so maybe he was afraid messing with the Borg was a sure fire way for his son to get kicked out of the Continuum. What do you think?
I think the Q, as a species, long ago tinkered with every permutation of the universe they could imagine, just like Annorax in Year of Hell on a much grander scale. Killing species, watching the repercussions, bringing them back. That's essentially part of what they mean when they use the euphemism of traveling the road and being the scarecrow in "Death Wish."
The difference between Annorax and the Q really being that the Q could literally undo whatever they had done and start over, while Annorax could not - he could only take additional actions that might happen to undo some elements of changes he'd already made.
Eventually, the Q settled on the current timeline, for whatever reason, as their preferred one. They therefore become guardians of that timeline by default since they see any other outcome as sub-optimal. Tinkering by anyone with that timeline has potentially dangerous repercussions for them if left unchecked, so they police each other - making sure that changes by a Q to the timeline are small and cleaned up quickly. Random, unstable elements like a relatively immature child in the continuum can become a threat, and why Q Jr. needs Q to be his minder.
As far as the son getting kicked out of the continuum for what he's doing (or tried to do), I think you're essentially right, following that same logic.
I'm sure there are gaping holes in my logic, but that's what I've gone with. That's why truly omnipotent characters in a story are a really bad idea. It's almost impossible to write a story for them that doesn't fall apart on close (and sometimes even cursory) examination.
