OP, it sounds like PO had reservations from the get-go, the way she has written her replies to you make it sound like she was willing to have you and co meet the pony and see if you were a good match. not to devalue you or your son, but high-needs/special-needs people require a very rare pony indeed - consider that maybe the PO was willing to see if your son & his special needs were an okay fit with the pony, NOT that the pony was a good fit with your son.
an example, as i have volunteered with therapeutic programs and my sister was heavily involved with one. there was one child that had aspergers, that grew frustrated when he could not express himself as well as he articulated and composed his thoughts innately. he was clearly a very intelligent person, but there was a barrier in expressing himself. he was not a suitable candidate for one of the best lesson horses in the program, who was an unflappable type. why? the horse, more a lab than equine, was one of the most emotionally intelligent animals i ever met – the child needed a horse that was not emotionally sensitive, as this horse had a very difficult time understanding the child. the child needed a ‘dull’ horse, a horse who didn’t care how you were feeling that day.
you have to understand too, that not everyone understands special needs people. not everyone is going to be comfortable selling their horse to a special needs family. it may not be politically correct, and it may not feel fair, but it is the PO’s pony and she has every right to refuse the sale.
who knows? you’ve been vague with what your son’s handicap is exactly, but consider this. PO may have a history with pony, hypothetically, lets say pony is great in every aspect but occasionally may need a strong handler while grazing, or loading. every perfect pony has a flaw (our perfect pony NEVER ran out of energy). lets pretend your son has ALS or some other devastating muscle wasting disease - maybe PO could see that it was not a good match for the pony, maybe she was envisioning your poor son being taken advantage of once pony realized your son wasn’t going to assert himself physically to stop her.
i’ve BTDT. it sucks, but it’s horses. nothing is guaranteed and you need to realize even if you show with cash and a trailer in hand the deal isn’t done until that pony is in your stall, eating your hay, and you have a contract in your hands.
also, if i had been the PO, i would have been very off put by your language. you do not come across as crazy IMHO, but some of your language seems desperate and short-sighted. as a seller, i want to know my horse is getting the best possible home - the buyer’s language is very important to me and i would screen heavily any person that did not have a fully supportive SO/husband. JMO.
this may be a good lesson for YOU OP, to learn not to count on other people to always hold to their word. also, consider this a lesson in politics – saying one thing, meaning something else. i think, in a way, you may take things at face value too much – realize this, there is an entirely secondary and different language in the way we construct our sentences that fully articulates what we really, truly mean and feel. what words we elect to construct our sentences does not always come at face value - learn to read in between the lines, as it was very clear from PO’s diction that she was making no promises.