First off, $900 for an autism diagnosis?! ZOMG. 

Is there anyone else you can be referred to?  You mentioned in the other thread that you're in Australia; whereabouts are you?  Have you had any contact with the Autism Advisor in your region? (every state has that scheme, I believe..)

"The greater my realisation is around my own ASD, the more difficult it is for me to understand the world around me.  Is that common?"

I don't know if it's common, but I can personally relate to that.  Sometimes I wish I never knew, because maybe then it would be easier to cope... but I guess that's a whole different discussion there...

"When someone comes off as disingenuous or inauthentic, I find myself becoming sad, disappointed and then eventually angry.  This is the point at which I tend to sever friendships." 

I can relate to this, too.  What has historically helped me in the past is talking to the person about it.  Hard as it may be to initiate such conversations, it's better in the end because - more often than not - my feelings of inauthenticity are completely off-base.  I hate to admit that I often read people wrong, and the only way to clear the air is to ask.

"I don't know if a dx would even help at this point, I'm 99% positive for myself."

A diagnosis would give you access to disability services, pensions, and concessions in your workplace, if you should ever need them (and you never know)... but of course, it's a highly personal decision either way.

"How do I maintain friendships with people who strike me as inauthentic and disingenuous when the world seems full of these people?  If I can't trust these people, who are my 'true' friends?  Are these people just being 'normal' people? My judgment and feelings about these people changes and I no longer trust them.  If this is 'normal', how do I ever develop true friendships that I feel like I can trust in my heart?"

Here's my take on things, FWIW: 

Everyone in the world is inauthentic and disingenuous to some degree.  Even ourselves.  We may not see it, we may vehemently protest that we're the very model of WYSIWYG, but I guarantee that everyone's got something to hide.  "All the world's a stage", as the famous Shakespeare quote goes.  We're all actors to some extent.

So, the real issue here is trust.  Who can you trust?  "Hubba-hubba-hubba-money-money-money-who-do-ya-truuuuust?" :-D  Sorry, channelling pop culture there... I don't know the answer, honestly.  Sometimes indescretions are easier to overlook than others.  Sometimes we have no choice but to trust certain people.  Sometimes the alternative is worse.

I could say something like, "Focus on the good qualities, assume people are generally good rather than generally evil", but I know that's also easier said than done.  I struggle with that, too.

I do know this:  all the long-lasting, true friendships I've ever had are wrought with ups and downs...feelings waxing and waning.  I get annoyed with them, I get angry, I feel distrust.  But the feeling passes, and it passes because we're honest with eachother about it.  We get it out there, we deal with it, we move on.  We don't keep it bottled up inside.  We don't let it fester until it becomes unbearable.  Unfortunately, I think this sort of thing is highly encouraged in our culture - particularly in women's circles - and it's a shame.  We've forgotten that friendships are hard work, sometimes.  We need to learn to forgive eachother. We have to care too much to let go.

Forgive me for waxing lyrical (I do that a lot on here!)... but hopefully some of it is helpful.  Cheers!