Hi TiredMule - We are back and had a fabulous time 
Great to hear that your dmil is getting to do some of the things she really wants to do.
I think, for the exit pass, you need your dmil to get a letter from her GP or hospital explaining her condition and giving details of why it may be more difficult for her to stand in a queue. You then take this letter to City Hall (at the entrance, just inside the park)
Have a look at this guide www.dlrpmagic.com/guidebook/practical/disabilities/
You may also want to hire a wheelchair for your dmil if she gets tired.
If you do not get the exit pass (I think they are called assisted access passes or something like that) then make sure you make good use of the fast passes.
My top tip for DLP is that the fast pass does not need to be used in the allocated time slot - you can use it any time after so just keep collecting them as you go through the day. eg. We got fast passes for Peter Pan first thing in the morning - the slot was 30 minutes later. We then did Dumbo and Snow White whilst it was quiet and there were very small queues and went back to Peter Pan an hour later.
We also got fast passes before the parades or lunch etc and used them afterwards.
You can collect the next pass as soon as your first time slot has come up, or in two hours, whichever is sooner.
Extra Magic Hours are great, but, if you have the staying power, it is actually much, much quieter in the park between 8pm and 11pm. We often found ourselves leaving the park at 3ish when it really is mad, going back to the hotel to use the pool or have a nap and then going back into the park later.
One evening between 9pm and 10.45pm we did 9 rides that normally would have queues of about an hour. We still used fast passes too so queues were even shorter as most people don't bother with them later in the evening. At 10:45 we found a spot to watch the fireworks - well worth it!
Weather was bizzare. It went from bright sunshine, to heavy showers and back to sunshine again at the drop of a hat.
I would suggest very comfortable shoes and lightweight waterproof coats.
We booked Buffalo Bill tickets through a French website - let me know if you want the details and I will look them up again - it saved us at least 20 Euros per adult ticket.
We had dinner on the first night in the Inventions restaurant (which is in the main Disney Hotel) It is pricey, but I think the best place to see the characters and the food is really, really good. About 8 or 9 characters came to our table for photos and autographs throughout the evening and it meant that our dc were happy not to queue for ages in the park for photos for the rest of our stay.
Let me know if you have any other questions, really happy to help. I feel on a mission to make sure you have the most perfect holiday!