There are probably a few ways to accomplish what you require.
- to be able to bulk upload multimedia files to a central library
Use a File System Repository and anything you upload to this file system will be available in the Moodle File Picker in any course which has access to that repository.
- not to have to adjust php.ini files
If php.ini has a high enough limit to start with, you can throttle the upload limits by course. See "course settings" for that.
- to have consistent file paths, i.e. the file path is always the same throughout all courses and activities
Moodle 2.x uses files differently than 1.x did. A file is considered "included" with a specific activity or assignment. In this case a "copy" of the file is made when you add it to an activity so the concept of "consistent file paths" is no longer possible. I saw some posts discussing changes to this to allow for updating of changed files, but I'm not positive if/when those changes will be available.
- roles based permissions for individual uploads, edits and deletions
Ok, when I read this I started thinking about your request in a different way. What you might consider is having the multimedia files hosted outside of Moodle... for example in a YouTube account. The current YouTube repository doesn't have fine control over this but I suspect an enhanced YouTube repository where each YT repository instance connects to an individual user's YouTube account would give you the permissions control you want. Users control their videos through the YouTube interface and then publish them in various Moodle activities through the enhanced YouTube repository.
Another thing to consider is using the Dropbox repository. This already has user-based access and can support any kind of multimedia file.
Not sure if this helps or not, but I saw your post and wanted to give some ideas.