Makes sense about your mom! And yes, I think you're right. If you're sitting at Tomorrowland Terrace at the Treats & Seats party, you'd probably be able to see the parade as it circles the hub in front of the castle, but at quite a distance. You could leave the party, go to Main St for the first parade (it's very close!) and then return to your dessert party (seated or Plaza Garden) for fireworks afterwards.
The parade... I've seen the Christmas parade 4 or 5 times and for me, it's all about the ground-walkers. The floats are good, but the Christmas parade ground-walker sections are my favorite things in any Disney parade. And I like the snow on Main Street.
Scooter note - the Plaza Garden Dessert Party fireworks viewing area has a dedicated section for ECV's that's really nice. You go to the Plaza Garden viewing area as you normally would. The CM's will see the ECV and will direct you to the scooter area. The scooters go through the garden, through a gate at the font, and basically park on the (roped off) sidewalk in front of the Plaza Garden. That sidewalk is fronted by another garden with a low wall, which means that no one can stand in front of the ECV user or block their view in any way. If you do the Plaza Garden party, your mom will have a stellar view and will be able to comfortably sit and chat with other ECV users and their families before the fireworks begin. The ECV user's family can stand there with them (you can sit on the low wall while waiting for fireworks to begin). Or, the ECV user's family can go stand/sit on the grass in the Plaza Garden grassy section. When the fireworks are over, the ECV user turns around and drives back into the Plaza Garden grassy area, then exits that way. Many dessert partiers choose to hang out in the Plaza Garden waiting for the crowds to disburse. If your mom is not super comfortable driving the ECV in crowds, she might want to do that.
Hope this helps. Whatever you decide, I'm sure you'll have a great time!