...So every EA client that makes changes to a diagram in that repository will automatically update the cache for that repository.
So when there is no change, there is also no need to update the cache.
And thus still no need for the worker process nor any PCS token.
What if a user deletes an element (from the Browser, not opening a diagram), and the element is used in several diagrams? so, you will need to update the cache for all those diagrams. Does it work in that way?
If I were the PCS, I would update the diagrams cache according to the date of modification, but at the end of the day I would update all the diagrams cache because all of them could be updated due connector or elements changes (edition, tags, deletion, renaming, etc)