I think that the key thing that makes Xander the contradictory mess he can be sometimes is that he's the child of alcoholic parents. I try not to buy in to the fanon presumption of physical abuse, because it was never even hinted at (that I can recall) in canon, but there is no doubt that both of Xander's parents were raging, dysfunctional alcoholics. To make sure my impression of the tendencies of children of alcoholics wasn't totally off base, I checked out a paper that serves as an overview of the current research about the tendencies of children of alcoholics (COAs).

Certain common traits were found in COAs without coping problems:

the ability to obtain positive attention from other people

adequate communication skills

average intelligence

a caring attitude

a desire to achieve

a belief in self-help

There were also other traits common to both coping and non-coping COAs:

lower scores in math, reading and verbal tests than children with non-alcoholic parents, despite having normal scores on intelligence tests

underestimation of their own competence

underration of their abilities by their mothers

higher levels of depression and anxiety, along with more symptoms of general stress

a feeling of a lack of control over their environment

(Please see the summary I linked to above for all citations.)

Damn, doesn't a lot of that sound like Xander? And more than just that, knowing what Xander needs to be a fully-functional person (as in, one without "coping problems") helps to explain why the changes between our Xander and AlternaXander are as pronounced as they are. For instance, it's pretty obvious that our Xander operates best when he's getting positive attention from others. But once you realize just how important that attention is to Xander's mental balance, it makes more sense that AlternaXander would cut ties and go build a new life with the one person giving him that positive attention. Turning his back on saving the world, or even just saving lives, only really makes sense for AlternaXander when we realize that he probably knew (at least subconsciously) that staying would have tipped him into the group of COAs with "coping problems." No sense in staying behind and helping to save the world if it you're going to be too damaged to be of help anyway.

Also, the tendency of both Xander and AlternaXander to handle things themselves ties in to their belief in self-help. And when I say handle things themselves, I don't just mean actions like AlternaXander's vamp hunting (which also can be seen as being driven by his need to overcome his feeling of a lack of control over his environment). You see signs of the importance of self-help for Xander all the way back to his insistence on going with Buffy to try to save Jessie. You don't let others do things, you do them yourself. Not necessarily by yourself, mind you, but you don't sit back and do nothing.

I've already gone on too long. Suffice it to say that this insightful discussion about how Xander came to be Xander, along with the brief research I did, makes AlternaXander a more believable possibility for me now. And I'm really, really looking forward to these promised chapters from his point of view, so I can see if my impressions of AlternaXander's motivations hold true.