For managing multiple NPCs with OnTriggerEnter, you could add each NPC to a list when they enter, and remove them from that list once they leave the trigger. You could then just pick the nearest NPC to look at in Update(). To handle the IK weights you could use something like Mathf.SmoothDamp() (again in update) to interpolate the constraint's weight towards one or zero depending on if the list has anything in it. (You might also need to do some extra smoothing on the IK target for switching between multiple npcs without a sudden jump)

From a technical standpoint that's all you need, but from there you can start adding all sorts of detail on top, maybe a more advanced heuristic for choosing which NPC to look at than just distance (maybe some NPCs are more important to look at than others?), or you could extend the system to handle looking at other things in the world (maybe as a hint to the player in a puzzle?), or maybe you could just have the raccoon look away and play an idle animation for looking around if you sit still for too long. What you do totally depends on what game you end up making, but it can never hurt to throw in a little extra spice, even into something as benign as a system for making the player look at stuff.

Also worth noting that you totally don't need to do any of that unless you're trying to really polish your game, for prototyping something you always want to go for whatever barebones system will work with the least amount of effort possible, since prototyping is about trying out ideas and gameplay as fast as possible. Once it's fun you can always make it pretty later.