I went to school with a boy who had one leg shorter than the other so he wore a shoe with a big thick sole so even though his height seemed even, his knees weren't even since his lower leg was withered. So if he was similar to Steven he'd be able to move faster he would have to adjust to the difference by hitching his leg
I don't think the behavior is odd at all. Their son was disabled physically but he wasn't disabled mentally. The accident that caused his disability happened in Edinburgh, Scotland when he was two. So for his whole life up until he went missing, his disability may have been the overriding identifier as he grew up. He may not have been able to access the care for rehabilitation while in a foreign country and ultimately experienced all the negative things that could have occurred in his life: the staring, the isolation, the diminished ability to play sport or going out on a bike ride with other kids, to go to a dance with a girl, going on a date, all sorts of normal things that kids growing up experience. Include in that, the moving from one country to another, eventually being physically comfortable in his space only to be uprooted to move back to the UK. It sounds to me that his parents were equal parts solicitous to his disability but also frustrated with the child they basically created: a child who may have been injured because of a momentary lapse in care creating guilt for his injuries. He received a fairly sizeable payout which was managed by his parents and as he aged he wanted access to those funds. He had a job but he wasn't amenable to shelling out his own funds for entertainment according to his parents which caused some arguments and I believe them. It seemed he wanted his independence but was tied to his parents physically, financially and emotionally and I think a lot of arguments occurred in that household as to the viability of him living independently. It's possible there was a lot of yelling and storming off about a lot of things in that household; two former cops who's jobs required adherence to rules and regulations who straddled rules and the relaxing of rules who battled with guilt, frustration and fear for his future, a daughter who seemed to be an enigma, an afterthought or an observer and a man who struggled with dependence and a desire for autonomy. I don't know what really occurred the day that SC went missing but if he stormed off out of the house and never returned for hours and hours I can see the parents concocting the story about the walk and him leaving the lavatories and the mom not seeing him after that because public opinion about letting a disabled man takes off in anger and not do anything about for hours comes across as unfeeling when it could have just been emotional and physical exhaustion. After 34 years, after being arrested for murder, having their property dug up then having charges dropped, if LE had any indication of their guilt they'd charge them.