I think you may be on to something with the stress levels. two things popped into my mind, not sure if Dr. Schwarzbein discusses them. (I've ordered her book and have yet to read it, hopefully I'll get to it soon)

One is PTSD, I know of someone who is a vietnam vet and there is a stress that is always with him. Sometimes stress that someone is experiencing today can be from something that happened a long time ago. For example an abusive childhood or past assault.
I'm not sure about this but I wonder if food allergies can cause a rise in cortisol. I know they cause a rise of adrenalin, I'm pretty sure this would mean a rise in Cortisol as well.

There are probably lots of reasons someone's cortisol could be high, I hope you figure it out.

I've never had it tested but I wouldn't be surprised if my cortisol is probably high too. A lot of my stress I've carried over from a difficult childhood.

I don't know why your insulin would be low, I wish I knew more. I would think about seeing someone in the know. Seems like something a naturopath may know about, or you could try the endocrynologist.