No, they cannot mess with your FP or ADRs. I was worried about that too! We overlapped trips with a friend last summer, so we did exactly what you're talking about. We became friends in MDE, and she gave me "permission to view and download pics." I downloaded them all and made her a CD.
The part I don't remember is whether I could download her edited photos (with borders, etc.), or whether I could only get the originals. I think I gave her my password, and she made her edits in my account. Maybe someone else can clarify that part.
There is also an option to give permission to "view all my plans" and "viewed shared plans" (I think those are the terms used). We turned OFF the "view all plans" because it was confusing seeing both of our plans for the same day. I guess that wouldn't be an issue if your weeks don't overlap at all. But there was absolutely no way for her to mess up my stuff, or me to mess up her stuff.