For these two, it really boiled down to insecurity and perfectionism. They wanted to be perfect, to be loved, and so they lied and were defensive regarding anything that suggested to themselves or others that they weren't perfect. They manipulated others as a power play and as a way to show themselves that someone loved them.
I like how this is phrased, and I believe this is the case for my friend as well.
(I just wrote out an entire paragraph and deleted it because I was basically repeating what you said but phrased more worse hahaha. Kudos to you SAHM!

)
I just want to add one quick thing though:
Do you think it's bad to feel this strong desire to "help" these kinds of people? I am always careful to not:
a) act under the assumption that I'm "better off" than them, and
b) get too involved in something such that I end up making myself unhappy.
But on the other hand, I consider myself a caring person (oh that feels braggy to type!) and I sometimes just want to show these people (especially peers) that someone cares! That I'm someone who DOESN'T need to be manipulated to like you, who DOESN'T need or expect you to be perfect, who will stick with you no matter what, and that you DON'T have to get all defensive or scared or "put up a front".
Opinions?
In addition, to those of you who have been discussing parenting, thank you

I have been trying not to allow myself to develop a bias towards only-children based off of the one friend that the original post is about. But it's been tough because I mean I can't just go around finding only-children and interviewing them about their childhood/parenting/IQ/etc.!
I really appreciate all these points about how important parenting is, especially from people with experience! (ie. actual parents!)