Wow. Sounds like a heavy-duty power struggle is going on here.
I know this is not the usual thinking, but try to consider 'going potty' as your son's challenge to master, not something you need to make him do. From reading your posts, it sounds like your son is under a lot of pressure from the adults to use the toilet. Santa's toys have been made contingent upon this, and then taken away, which must feel totally disempowering and discouraging to your son. ("I must be bad if Santa took away the gifts." is what he's likely thinking.) These are all big no-confidence votes to a kid.
I've been helping children learn to use the toilet for nearly 20 years, as a professional nanny, toddler-age and preschool teacher. I'll share a website with you that I think will really help:
http://www.betterkidcare.psu.edu/AngelUnits/OneHour/Toile....
The primary philosophy of toilet learning is to make this HIS process, not yours. I would scoot away from punishments and rewards, and just give accurate feedback. With his ADHD, he will also need a lot of help getting to the toilet. Don't ask him 'if' he needs to go, but do tell him every so often "It's time to go to the toilet now". Find a way for him to separate from what he's doing, and make sure he gets into the bathroom. At this age, it's tempting for us to tell them to go potty and just expect them to follow-through, but they still need us to help them. I find playful invitiations helpful too. If he's playing with a train, "Let's be trains and chugga-choo-choo to the bathroom now" can help. It becomes an extension of his play instead of conflicting with it. A little imagination can make a lot of these transitions a bit easier for everyone.
And no, you aren't alone and you aren't losing your mind. I worked with families whose very typical children had issues such as these. Two concrete things that helped were 1. observing and recording what time of day the child was having 'poop accidents' and 2. using their body's schedules to put them on the toilet a bit earlier than when the accidents happened.
I also made sure that when accidents happened, that they didn't become solely my problem. I expect children to help clean up. If it's pee, I expect them to change their own clothes in the bathroom and to make their clothes laundry-room ready by putting them in a plastic bag. If it's poop, they need more help (children should not be handling feces), but I also had the child help me by staying with me, having them open doors for me, hold bags open, etc.
Give positive feedback when its warrated, and realistic feedback when accidents happen. "I see you pooped in your pants because you didn't want to stop playing. We'll have to take a break and go potty earlier next time."
Hang in there-- you aren't the only one, but I know-- there are times when I thought toileting a child was the most thankless task in the world.:)
H.