I don't know how much I can say about this but I'll try. I've had no experience of Alters, so to me that sort of behaviour just seems as incomprehensibe as it likely does to you, indeed "childish" in it's most literal sense.
With isolating however, one thing at least for me is that people take work. I can deal with people, indeed I can get on stage and perform at people or give a lecture at an international conference, indeed using social skills and applied empathy to get people on side is one thing I'm good at and if you met me in person doubtless you'd think I was an absolute extravert and loved others company. Indeed, one counsellor I met for a first meeting, after a half hour of pleasant conversation, mild humour and friendliness actually asked me "why do you need counseling?" and it was only at that point that I realized what I was doing and forceably stopped.
This isn't to say it's false or putting up a front, it's just applying a set of learned skills in a given situation, much as someone who was an expert chess player but awkward at goodness knows how many other things would be a very different person across the chess board to how they'd be the rest of the time.
Given however that (honest though it is ), for me, interacting with people is a use of carefully learnt social skills and empathy, it is work. There is no other way of saying it, it's actually tiring!
to relax, recharge or whatever, for myself and doubtless for other intraverts too it's being alone does it. Being alone there is nobody to need to practice those skills for and I get to just be on my own resources, enjoyingreading, writing or whatever.
I've actually noticed quite often among couples one will be intravert and one extravert. this is certainly how my parents are. My mum, like me can apply! all the social skills and empathy, but in order to actually relax needs to be alone. My dad on the other hand actually enjoys! other company and will actively seak it out, indeed it's that something of a family joke that if my dad goes to the shops or similar and doesn't come back for a while we say "oh he's met someone to talk to"
Of course however when you add in abuse, that certain amount of necessary isolation to relax can get twisted, ecause isolation is for an intravert quite addictive, and I've certainly been guilty of this myself, not speeking to others for four or five days straight.
Of course I did realize this, and while I will admit I don't always take dealing with people with good grace I do try since I am aware how easy it is to get overcome with the ease of not dealing with anyone, especially when there are other things, phobias, depresison etc, not to mention apathy which get in the way.
As regards overscheduling, I know for me at least that would be one of a few things. Either it could be simply not noticiing how much I was taking on through having my mind on other things, then suddenly realizing. Also however it could be an attempt to do things for others. I know myself it is much easier for me to do something for another person than myself, thus if you asked me to do a job I'd likely take it as a complement and work to the best of my ability to do it, even if it wasn't practical for me to.
Equally, it could be as you said, another form of isolation.
I do hope some of this is helpful. Of course this is most often based on me, and thus I don't know how much applies elsewhere, but still I hope you some of it gives you an idea.