Can non-carbonated energy drinks boost alcohol intoxication?
A very well-written question, so I'm sorry if my answer is a bit too simple. Perhaps it will attract attention of somebody with a better answer.

I think the effect could be mainly caused by the fact that sugary drinks slow down alcohol absorption. So while those who drink just alcohol already know they've had enough, those who have had a couple of energy drinks still feel fine and continue drinking, not knowing that they are about to become even more inebriated.

Also, the stimulants in energy drinks can somewhat reverse the sedative effect of alcohol, leading to higher overall activity of the person and thus faster drinking.