MrB I've never used that and didn't know it was there. Crazy. Been using it for ages too.
Well, as far as I can see you stumbled on a bug which is never a bad thing.
Realistically then, in your case, the move forward has a bad step.
I think as it stands the program will not be able to do it out of the box. Whilst roles allow for manager restrictions. It doesn't allow for document restrictions as before and you will be either to edit or not on everything.
There may be a way to have a system where you will be able to achieve the same or similar affect but in my case, I'm not sure on which bits to attack in order to do it. It wouldn't be for the average site builder. You certainly have a need for it.
The only way I can think about it at the moment is giving all pages a TV and assigning a number to it to represent the user who can edit (or maybe access group name to represent a group of users) and have all the docs as public.
Then maybe a plugin that checks the user and group against the document being edited and allow if it matches. Maybe an Evo guru could say whether this is a way to go. Sadly it would mean a lot of work in populating the document TV unless there's a programmatical way to do it. It's quite a task for 30-40 sites. There's nothing stopping you having user groups.