Explorer tree for timeline that accommodates items being added/removed over time

I'm developing animation software for a desktop application. What I'm concerned about is the lower timeline and explorer which show the composition of the animation. Here's a mockup:

Depending on what the user is doing, the animation might need to have components added to or removed from it part-way. In this reload animation for example, a second magazine is brought into play and the first is removed from the animation while it's still playing. Maybe that's not what's going on in that specific video, but let's say a user would like to be able to animate that way.

The lower timeline, of course, needs to reflect that a component was added/removed. I could have the component only show up in the explorer on the frames that it's in the animation, but perhaps the user adds a component, forgets about it, and then can't figure out why their animation is behaving unexpectedly until they scroll over to the responsible frame. I thought of making components always visible in the explorer and scratching out areas on the timeline they were removed like so:

but if an animation already has lots of components, displaying even more in the explorer at all times would just make it even more cluttered. The software in question needs to have a low learning curve, as the users of the application have never animated before. How can I communicate component removal/addition to the user in the explorer/timeline in a manner that's both easily visible but not information overload?