I think its extremely likely...I've been in a psychiatric institution and have been told that I am hi polar. I think growing up in a cult is likely to exacerbate any underlying mental disorder and trigger episodes of illness as you say teens are especially vulnerable.
The chicken and egg problem is do cults produce mental illness or are folk pre disposed to mental illness attracted to cults?
As regards the GB they have so much invested in the cult now its too big of a u turn for them to face reality...but yes, believing BS that you yourself make up (ie I am special and different, gods chosen one etc) IS a type of psychological illness or malfunction if you like..those guys need an " intervention" LOL!
When will families start having " intervention" for their witness relatives the way families of addicts do! Roll on the day!!