Hi there, in terms of task reassignment, have you looked into task/issue delegation? In Setup > Project Preferences > Tasks & Issues there is the option to allow delegation. More on this functionality here.

For proof ownership coverage, take a look at this, but mind you it's only within the proofing area/site.
I'd do a task delegation and have the designers that could ever take over set to a supervisor proof permission level in their profile. This will allow them to upload a new version of a proof that they aren't the owner of/didn't originate - other proof permissions won't allow you to do that.
All I can think of for now, good luck!
If this helped you, please mark correct to help others : )