Arielilane is simply doing what we are all doing: drawing conclusions from circumstantial evidence. It's all anybody can do, since the most important eyewitnesses are all dead, except for one very little boy who doesn't remember.
After rereading the last couple of pages, I think the simplest explanation is that Diane speaks with her brother, lays down the phone and immediately forgets about it. She then takes a wrong ramp and heads north instead of south.
She is still drinking and/or smoking dope so her comprehension continues to deteriorate and it is some time before she realizes her error. She may also be distracted by whatever caused the heavy drinking in the first place. (IIRC the highways in that area twist and turn quite a bit. Anyone could be confused as to north v. south, but a drunk would be especially likely to be befuddled.)
When she finally realizes her error, she takes the next ramp to go back south. Unfortunately, in her drunken state, she confuses a north-bound ramp for a south-band ramp and is now driving against the flow of traffic. (The video shows how easy it would be for an impaired person to do so.)
The kids see her error and are screaming. Diane panics and drunkenly decides the best course is to go as fast as possible to get to a place where she can get off the highway. This is of course fatally stupid, but she is too impaired to think logically. So she races the wrong way in the fast lane, honking to ward off oncoming traffic while looking for a place to get off the highway.
With the other car approaching at 75 mph and Diane going 80 (while looking for an exit), she may not even see the car she hits.
Occam aside, the simplest explanation isn't always the accurate one. But I think the above is more likely than a conscious murder/suicide by such potentially ineffective means.