Hi Cynthia,
Starting your vacation with the Magic Kingdom and a delicious meal at Kona Cafe followed by a viewing of
Wishes Nighttime Spectacular sounds wonderful!
I checked, and currently Wishes is listed as an 8:00 p.m. showing for your first night. Therefore, I recommend that you stay at Disney's Polynesian Resort and watch Wishes from the beach. One you have finished your dinner and wait for transportation back the Magic Kingdom, I think that you will be very rushed. Also, the later you get to Magic Kingdom, the harder it is to find a good spot from which to watch the show.
Typically, I recommend that the nighttime shows be viewed from within the theme parks; however, Disney's Polynesian Resort beach is the exception. You have a straight-ahead view of Cinderella Castle and the fireworks overhead. On a recent trip, my family watched the fireworks from this beach with fellow Disney Parks Moms Panelists Stephanie, Jacqui, and Kirsten and their families. It is truly one of my favorite memories from Walt Disney World. Do be sure to check the time for Wishes as you get closer to your vacation as it is subject to change. You can do this by clicking on the link I gave and then changing the date.
As to your mom using an electric conveyance vehicle (ECV), I think that she will be fine to get to the beach at Disney's Polynesian Resort. There are sidewalks right up to the beach. I am not sure if she is able to transfer out of her ECV and navigate sand at all, but if she can walk a few steps on the sand with your assistance, you could pull up a beach chair right to the sidewalk at the back of the beach an you would still have a wonderful view.
Have a great time planning your trip, Cynthia, and please do not hesitate to ask if you have any further questions.