Getting on the bike is a little difficult. But I have cerebral palsy and can't really get my leg over (wheyey). But Ill try it again when I am home from work maybe see what it will be like with a shorter stem. Maybe make a couple of adjustments to seat height and whatever.
Otherwise it's probably better off buying a new bike? Seems like such a hit to take on a bike that has hardly been rode. Bike resale value sucks lol.
for a mountain bike, it's generally better to ride a bike that is a size too small, than a size too big. Considering that you have some mobility issues due to particulars with your health, I'd say that's especially true for you.
A bike that gives you a maximally efficient pedaling position (assuming perfect health and mobility) won't do you any good if it's hurting you somehow or is difficult to get on. First and foremost, you want to be comfortable on the bike. Nonstandard modifications are also a possibility to accommodate specific issues you might have. I have a friend who recently went through a similar process with a bike fitter and a physical therapist.
Kudos to you for getting on a bike in the first place with cerebral palsy. I have a friend with the condition who would love to ride a bike, but hers is severe enough that her balance is heavily affected and she would need a trike of some sort. Either a recumbent trike, or possibly a handcycle. And she just can't afford those.