By what I'm reading, IPE was the root cause, lack of immediate bailout source was the fist broken link in the chain of events that led to the outcome.
I can see difficulty breathing which led to the wise decision to bailout to something that breaths easy (OC) and if some uneasiness/panic was already setting in, either by the difficulty to breath alone or that combined with the issues with the DSV, and maybe the nature of the entrance to Cow (haven't been there much or recently, so this may not mean anything), diver is turning for the exit, abandoning reel/line while simultaneously deploying long hose on right side cylinder which gets caught on line and entangles to a point where he is unable to reach his mouth, then we can imagine panic being much higher at this point that deploying the left side reg led to the issues with that bottle...
The entanglement is a bit weird though, thinking that the line ends right where he was and not being tied in it's a loose end, although entangled in it, should still be able to connect hand(reg) to mouth, first thing to come to mind is, if my hand cant reach my mouth, then my mouth will try to reach my hand, easy to say it from the chair I'm sitting in of course.