1. as far as I know, restore points dont just get deleted, but you can delete them by doing a Disc Cleanup and going to the “More Options” tab and cleaning up System Restore points. its pretty easy to figure out.
2. yes, a System Restore will undo registry edits and any other changes you’ve made, but most of the time it wont delete files you’ve created.
3. April through July might have been cleaned up by someone doing a disc cleanup (like I mentioned above), otherwise who knows, i’m not sure how any automatic deletion process works if at all
4. no, changes made by users dont revert after the user is deleted. anything in their user directory will be deleted, but any system changes they made will still be in effect