Do you have a preferred room location at POP, right now? For $15/day you get to have a resort room closer to the lobby, food court and buses. Generally, this means you are in the 50s or 70s buildings. This is not a complete answer, but it does help to a degree.
You can get a wheelchair rental for the resort and parks. This does not have to be used all day, either. This way you can have your mom sit in the wheelchair for the bus travel and getting back and forth from the parks. Of course, this means someone will have to push it. Most park stroller rental locations (who rent the scooters, too in the park) will allow you to leave your wheelchair there at the beginning of the day and pick it up when you are ready to leave. Just make sure you get a receipt. If you want to keep the wheelchair with you your mom can use it as a walker (stand behind it) and not ride on it all the time. These options are cheaper and better than upgrading to a deluxe resort.
If your mom fears using a scooter have her practice at her local stores for free. You can even go to a local zoo for a day and get good practice that way with a scooter. I agree with the other posters that it would be better with bad knees to use a scooter. You, yourself can get the scooter on and off the buses (hardest part for most people) for her. You can rent at the parks (for more money) and then you don't have to deal with the scooter for the buses. Your mom will still have to walk at the beginning and end of the day to the buses, but it should be better and easier if she has used a scooter all day at the parks. Keep in mind using a scooter all day at the parks will probably give you a longer day and a better, faster day. You will likely move faster and get to more attractions and so forth. Just help your mom with the scooter. You may have to park it for her or go through a ride queue with it while your mom walks depending on how mush she can handle twists and turns and reversing.
The monorail resorts help to a degree, but it can still be a long trek from your room to the monorail; the difference is you are inside the resort, but you still may have to walk a long way to get to the monorail. Exiting the parks you have to go up steep ramps to get to the monorails; afterall, they are elevated. Also, the monorails only help with 2 parks, not 4. Contemporary is not the only resort with monorails. You can look into The Polynesian and Grand Floridian if you are really set on it.