Thanks for the feedback. Handling deletes in a tricky issue. If something is purchasable then we'll push back and try to work with the developer before they can delete it. We do provide them instructions about how to contact all the owners about next steps before we let people delete paid projects. This is a manual thing though, and not handled through an automated system. It's pretty rare when this happens so we're happy to work with the dev to get it sorted out.
Regarding the "black rectangles," this problem exists outside of just deleting. Developers are free to do whatever they want to their page. I think maybe a broad solution would be for someone who owns something to have the ability to archive or remove a purchase from their so it's not shown directly in their library.