Hi @geeksmakemehot (interesting screen name... :-):
There may be a plugin to meet your needs but I haven't done the research so I don't know for sure if there is or is not. Hopefully there is and you'll get your needs meet for free.
If, after research and help from others you find that what you need is not available off-the-shelf, you might consider having us do it. We are building a platform extension for WordPress we call "Sunrise" to address business content management system needs such as these (with plans to open source it once it is mature enough) and we've already done a lot of similar work for our sponsoring client's needs.
Ironically I am working on a ticket today to implement essentially the same functionality you are asking about, except it will assume the site was implemented using features of Sunrise and you'll certainly want standalone functionality. What that means is I've already got my head around what's required to make what you want happen and it can be a bit tricky. WordPress doesn't provide developers with a clear cut way to identify "a certain page" nor "a certain parent page" besides a page's numeric ID, and it's not reliable to hardcode a page's ID into a plugin; what if one of your users deletes a page and adds another one? Now your functionality is broken.
If you want the functionality to be reliable you'll need to add a UI layer on top of WordPress to allow admins to specify which pages are editable and by which roles. There are a variety of techniques for that; which to use depends on your use-cases. The same is true for your category limitations.
Anyway, good luck finding that plugin, but if you need it developed then you can get in touch with me here. Be sure to let me know what your deadline is if you do.
