That definitely sounds like fun. I went on a trip with my mom last September and we both have mobility issues so I totally understand your concerns there.
I think your idea of watching Wishes from a resort is a good one. In September, mom and I watching Illuminations from inside a restaurant at Epcot and even though it wasn't the "best" view (and I have never had the patience to stake out the "best" spots anyway even when outside) it was really relaxing to sit at the window and watch the show. They piped the music in and turned the lights down...it was really nice. It was also nice because we took our time leaving the restaurant and didn't have to be part of the mad rush to get out of the park. Yu don't always have to beat the rush out...sometimes you can hang back a little and just sit on a bench or poke around a few shops while the crowds leave. Might mean some waiting in line for buses at the end...but anything is better for people with mobility problems than that mass exit at the end of the day.
Have you given any thought to having a dining plan...or (if not) on how you want to dine while you are there? I ask because with mom and I needing to go at a slower pace it was really nice to be able to take breaks and eat at TS restaurants. We sort of made it our big deal of the trip to try a bunch of places we had never eaten before...including dining in the castle which was a big highlight for us even though we are both adults. Because of this, we decided to go with the Deluxe Dining Plan and then stayed at a value to keep costs down.
Regardless of where you stay (and I was thinking maybe POFQ might be good since it is more compact than the other resorts) I would make sure that you call and request something that will not have a lot of walking for your mom. IE...first floor (so no stairs) or close to the elevator...something like that. All the resorts are different so maybe once you decide where you want to stay then you can ask on the resorts board where to request for someone with limited mobility.
Also, and I know this is a personal choice so I am not against the idea completely, but I would really consider about whether you and your mom want to do a split stay. The idea of having to pack everything up, re check in, etc might be a bit much. I don't know what your mom's limits are...but I would discuss it with her to see if this is really a good option. And if it is, maybe decided if you want to change over at the front end or back end of the trip. Instead of the cost on the deluxe, maybe you guys could spend that money on La Nouba or a spa treatment or something. Something that may be more relaxing for your mom.
Do you guys like shopping or shows? I would try to focus your trip on some of the things that you enjoy doing but don't always have much time for because of the needs of everyone else. If you like the shows but can't usually fit them all in, then this would be a great time to plan on seeing them. We did this at Animal Kingdom as we had never had a chance to see Festival of the Lion King and Nemo because we were always so busy. I also know when mom and I went we were really able to explore the World Showcase at Epcot more because we weren't all about the rides. We did our favorites of course and whatever else we felt like...but it was really fun to just take it slow and go at a pace that didn't wear us out, you know?
I like some of the non park ideas you have too. Doing things at the other resorts is a great way to see a lot of stuff at Disney that you just don't always get a chance for when you go as a bigger family group because you are trying to make everyone happy.
And just one more thing that mom and I really enjoyed...well...no...two actually because I just thought of another one. First...photopass. We got so many pictures together using photopass on our trip and it was one of my personal splurges that I pre-ordered the CD. Don't know when we'll get to go again together and it was really wonderful to have so many pictures of both of us. And second...the character meet and greet spots. We went to the Epcot Character Spot and it was SO FUN! I haven't had my picture taken with character is at least 10 years and we really just had a blast with that. Plus, on the first day we actually stood in line (only 10 minutes) for pictures with Chip and Dale. It was sort of silly...but Dale kissed me a bunch of times and mom got hugs from Chip...it was actually a special memory from the trip. Don't know if it interests you at all...but that was something I didn't plan for that we decided on the spur of the moment and it was a lot of fun.
I've never been to HDDR so I can't speak to that...but if you're young at heart I think that's all that matters at Disney. I do know we REALLY enjoyed eating in the castle though and getting pics with the princesses. The food was good and it was just magical to be eating in the castle. We got a table right by the window overlooking Fantasyland...it was awesome.
But no matter what you decide to do, just have a great time. As much planning as I did for our trip as far as which park to go to on which day, dining reservations, night shows to see, etc...I think we had a great time because we knew we were going to have a great time together. Have a great trip!