i am not a medic or psych of any kind, just giving my view based on years of running a pre school, nannying many pre teen kids and my psychology studdied over 20 years ago.

many high functioning/intellectual kids are mis diagnosed as ADHD. this is because they are bored in class so mess about, shout out answers to the annoyance of the teacher or find other annoying ways to try and let the teacher know they are not interested or are done waiting for the others to catch up to their thinking speed. Although highly itellectual these kids often lack the understanding of how or why they feel how they do so often can't tell the teacher they are bored.

being told mum is bad yet seing mum is good will really have an impact on his thinking/view of himself. he could be doubting himself and his views/thoughts. usually the adult caregiver is the best thing since sliced bread and knows everything in young kids eyes, to have someone consistently saying otherwise will make the child doubt their ability to judge others hence his lack of social intergration
often kids do things for a reason that is not visible or clear to an adult, tying something around a kids neck could have been your son pretending to be a cowboy or something and using the other kid as the horse or it could have been the kid was doing something your son disapproved of so he was trying to move him away or any one of a long list of things, the only way to know why is to talk to your son and ask if he remembers why he did it, same goes for the barbie and the kitten and any other 'weird' stuff he does/did.
until you understand why he does things you will not be able to help him.
if possible try restricting video games to use as rewards for spending time talking to you about what he has done and how it made him feel. start off with five or ten minutes when tv is off and there are no distractions, let him know you love him to bits, that he can tell you anything and you will not get cross or punish him for what he tells you, and that you may even be able to help him find a way to fix any problems or things he doesn't quite understand. next night spend five or ten minutes again no distractions, sit him on your knee if permitted, talk about good times from his younger days, ask if he remembers ....if yes then ask how he feels about it, was it fun, were you sad etc. acknowledge and confirm his feelings are/were ok and valid, from then on try every night to build up time talking with him ask more leading questions till you understand what is going on in his mind, over time you will catch up to present day things then you can let him take the lead by simply asking what did you do today? and how did it make you feel. by doing the reaffirming work before this stage he should be ready to more open with you than a lot of pre teens whose favourite words are nothing much and ok i suppose!
the pant wetting could be a sign of his insecurity in the world as a pre teen, sometimes people who have had a tough time enjoy regressing to a time they felt safe and loved, this is a way of self nurturing/comforting. there is a whole fettish world of people who love wearing and using diapers for their intended purpose ok some do it for sexual self gratification, but others do it simply because they make them feel more confident or better about themself. you say he wets because he likes the smell/feeling, this could be a sign his inner child is insecure and needs some nurturing...mine was screaming insecurity at me for a while, by me talking to my inner child and letting her know i understand she was upset and by doing a few very young childish things in private every now and then for a while, everything settled down and now my inner child is usually pretty content. i say this because your son suffered a trauma in your break up from his daddy, and in that he probably grew up in some areas faster than most kids do hence an imbalancce between his inner child and the child he had to be/is now. this is not that uncommon in split parent kids. some cope better than others with this.
a lot of things in your post seem to lean towards asperges to me, the high functioning, lack of tears, lack of social skills, lack of understanding of social etequet etc. has this ever been discussed?
does your son show remorse when he has done something wrong or get cross/stroppy if things do not go how he wants/ thinks they should?
like i said at the begining i am only going off experience and 20yr old learning, not professional qualification, so i may be way off the mark. if i am i do apologise.
take care x