If DEA internal-affairs were investigating Walt's confessional video, they might have some circumstantial evidence to support it:
(1) Hank's recovery was paid for by Walt
(2) Hank's neglecting his DEA duties and has already been reprimanded for this
(3) Hank declined a promotion to remain in New Mexico
(4) Hank was targeted by a rival cartel
(5) Witnesses can testify that Walt was in Hank's office, and Hank closed the blinds twice
(6) Walt and Hank were driving around together to the Chicken Place while Hank was in the final stages of recovery

None of these are strong, but enough for the show's writers to make something of them if they chose to go down that route.

I don't think his character was let down by the writers. In the past few seasons, he's mostly been a way to show us what Walt has become (I think the times I hate him the most is when he so obnoxiously lies to Jesse). But I really believe they have something great in store for him (as hinted last episode) and he'll have a good payoff.

There was a scene a few episodes back where that night came up - I think it was when Walt was talking about the irony of meeting Jane's dad in the bar - and I thought Walt might slip and say the wrong thing and Jesse would figure it out.   It's still a possibility, I think.

The other option, like you say, is that Walt just tells Jesse about it for some reason - maybe in a moment of anger or to rattle him, who knows.

I'm not completely sure they'll do anything with it, but it's a pretty big bomb to leave unexploded. 

As a quick follow-up...since the start of the 2nd-half of the final season I've been saying that the "flash forwards" would be caught up to with 2 episodes left. His return would be the 2nd to last episode, setting the stage for the climax and epilogue in the final episode of the series.

After this episode, it seems even more likely to me.

Next week's episode is titled "Ozymandius"...which if you do a quick Wiki-search on the famous poem, the first sentence in the Analysis section is this: "The central theme of Ozymandias is the inevitable decline of all leaders, and of the empires they build, however mighty in their own time."

I'm guessing that whatever the outcome of the shootout is, this next episode will be Walter's true fall from grace. Considering he was sitting in handcuffs and humiliated by Hank in this one...I can only imagine how far he's going to fall next week.

I'm not watching the show, but reading all this I have to wonder how much better this is going to be for future viewers watching it in daily syndication. Imagine all this happening, AND YOU DON'T KNOW IT'S THE FINAL EPISODES! I had that experience with Blake's 7, where every cast change and death was a wonderful surprise. I wouldn't be shocked if the creators are putting these episodes together with that in mind, that in the future viewers streaming the show will not notice there's no next season until they get to the final episode.

My only theory about how Hank/Gomie/Jesse walk away from this gun battle is that they surrender and Walter bargains with Todd's crew -- saying he'll go back to cooking for them if they let the others live, but if they kill them then Walter will never cook for them again.

That's the only feasible way I can see the situation playing out without all of them dying.

Any other ideas?