1) a formal XML schema document for ePO-exported XML policies
I'm not aware of such a document - I've had a look internally and can't find anything applicable, sorry
2) any guidelines for best practices when editing ePO's XML policy documents
Strictly from the point of view as a tech support guy, I'd have to say "Don't!" We do not support, or recommend, this kind of policy manipulation, although of course I can see situations where it would be valuable to be able to do this. I haven't investigated the ePO 4.6 web API in much detail - it may be that you can use this to achieve your goal, and still remain in a supported environment. Possibly some other forum members can offer insight - alternatively there's a sunforum for web API topics.
Thanks for the answer! I'll make an effort to refactor my approach to use the web API, instead of using file manipulation.