Won't this not quite work; because targets are already set before this resolves?

Yeah, it won't switch targets. The spell would have to be used proactively. That leads to anything being cast or activated would just not be cast/activated. You could play this before triggered abilities occur, I think? Is it even worth keeping at that point?

I think it's probably best to see if you can find wording that does let it work.

Or make it an aura instead of an instant, and accept that you have to do it in advance.

Instant granted the ability only until the end of the turn. An aura would persist as long as the enchantment remained, which I wouldn't mind, except that feels a little powerful for an uncommon spot.

Well, I mean Coalition Flag seems to think it's no big deal. Admittedly, it's been a while, though.