why wasn't Gosling better prepared before his meeting with Bernie? If I'm him, I'm going to that meeting expecting to get stabbed or shot.

That bothered me at first too, but then you have to remember, even though we saw Bernie stab a guy with a fork and knife him repeatedly and then later calmly kill Shannon, Driver didn't see those things happen or know that Bernie himself had been one to kill Shannon (he probably figured that Nino had done it, which is why he decided to take him out).

Whenever Bernie was around Driver in the earlier parts of the film, Bernie appeared to be a pretty level headed and genial guy. He did not seem at all like the type of psycho who would just go off and kill someone with his own hands (which is why his first killing is such a surprise and why Shannon is so stunned when Bernie slices his arm).

Driver probably figured that Bernie was going to send his hired goons after him and finish him off later, not that Bernie himself would suddenly attack him in a parking lot with a knife. And Driver wasn't totally unprepared as he was able to quickly defend himself and kill Bernie. But I'm sure Driver wasn't expecting to get attacked at that moment when he was just about to hand Bernie the money.

I honestly thought that the Driver went expecting to die. Kind of like Travis showing up at Sport's at the end of Taxi Driver; he showed up to take care of business. I assumed that he knew it was coming sooner or later and he didn't really care as long as he was sure that Irene and Benicio were going to be ok. That's the impression that I got from the note that he left for Irene. "Going away for a while," that to me said it all. Whether he lived or died he was never seeing Irene again.

Driver literally stomps the guys face into silly puddy in the elevator?

I agree. He's going in with the full intention that he (and Brooks) will die. There's the scene where he suggests to Irene that they could take the money and run... in the metaphorical sense he offers for her to get into the backseat of his car with Benecio and for the rest of their lives he'd be their "driver" protecting them. I'm pretty sure he knows that her accepting that deal is a long shot and also he likely knows the safest thing for him to do is end it in the fashion that he does. It's a noir flick, there is never really a way out until death.