Alberta DCPD - reversing into a parking stall, adjacent stall opened their door into me, insurance put me 100% (from 0%) at fault after other parties insurance denied liability.

I was backing into a parking stall in Calgary. The stall next to me already had a vehicle parked in it (nose-in). My forward-facing dashcam shows me reversing, pulling forward slightly to straighten up, then continuing to reverse the last couple of feet of the bay.
By that point my tailgate had already passed their driver door. While I was finishing the reverse, the driver of the adjacent vehicle opened their door into my path. Contact was with my rear quarter panel (not the absolute back bumper) and left a vertical crease on their door.
If the door had already been open, I would have hit it during the initial reverse or while pulling forward to straighten up. Contact only occurred on the final reverse after my tailgate had already passed their driver door.
Dashcam audio captures me asking why they opened their door when a car is reversing right beside them. They responded “how am I supposed to know.”
I have the video plus photos of both vehicles’ damage. The impact location looks more consistent with their door swinging out into me after my rear had already passed their door, rather than me reversing straight into a door that was already fully open and stationary.
I also filed a police report the following morning. The officer at the station said it was a clear-cut “dooring” scenario and that the other driver became liable the second they touched their door handle, under the road traffic rules about ensuring it is safe to open a door.
Originally my insurer was reviewing it. The other insurance company then refused liability on the grounds that I was backing up and the duty of care was on me. After that, my insurer changed the decision to 100% my fault under Rule 19.
I’ve challenged it under Rule 20(1). Contact with the side/rear quarter of my vehicle (rather than a pure rear-end into a stationary open door) shows the door was not opened in a reasonably safe manner and created a hazard to moving traffic. Rule 20 should apply, not just Rule 19.
It feels completely unfair to be held 100% at fault for something I didn’t cause, solely because my vehicle was reversing at the time.
Has anyone else in Alberta (or anywhere with similar no-fault/DCPD rules) had a collision where someone opened a door into you while you were backing into an adjacent stall? What was the final liability outcome after the other insurer refused? Did the video, audio, damage pattern, or police comments make any difference?
Any advice on how these usually get resolved would be appreciated.