To be honest, I never saw the child abuse of Diane. I heard the theory before, but I didn't understand where it came from.
Then it all clicked when a commenter mentioned it in that Hulk article. That person referenced the audition dialogue as the evidence and it makes perfect sense now.
Here is the dialogue:
Get out... Get out before I call my dad.
He trusts you... you're his best friend.
This will be the end of everything.
What about you? What will your dad think
Stop... just stop! That's what you said
from the beginning. If I tell what
happened... they'll arrest you and put
you in jail, so get out of here before...
Before I kill you.
What also makes sense is that a person with a history of sexual abuse (or really any traumatic event) will play the episode out in her head over and over again either consciously or subconsciously. In this case, we see her play this out twice in her dream - first while practicing with Rita and the second time in the actual audition as she turns the table and acts more dominantly. It's pretty damn creepy and terrifying to think about.
That brings me to who was the possible abuser, I think the two old people who walk Diane/Betty out of the airport are the possible abusers. We know they were with her when she won the jitterbug contest. Then we see Diane/Betty with them at the airport being very caring and parental with her. Then we see them in their own car acting extremely creepy and sinister. So we know they are actually from Diane's past and her subconsciousness is revealing them to her as having a sinister side to their bubbly caring nature at the surface.
Furthermore, the next time we see them is when they crawl out of the blue box at the end, which was possessed by the dumpster man. If the blue box is the portal back to consciousness for Diane and the dumpster man represents the horror and immense guilt of Diane's crimes, then these two old people crawling out of this reality portal are the sexual abusers revealing themselves to Diane at her most vulnerable moment since the abuse happened. These ghosts chasing after her compounded with having to confront her own crimes is what forces her to finally commit suicide. It's upsetting to even write these words. But that's how I view the film now.
Vulture put out this interesting piece last Halloween season and I agree with it. This is a legit horror film.