Yes, Walt obviously survives this shoot-out, but the ricin for himself? No way. First we have to get to the point where Walt is suicidal, which isn’t totally inconceivable, but with his cancer returning it doesn’t add up. Long life isn’t in the cards for Walt as a free man or behind bars, and if he gets to the point that he’s so guilt-stricken he wants to take matters into his own hands anyway, why would he want to do it in such a complicated (it was obviously a risk to even come back for the ricin), painful, and slow way?
Now I still don’t have a good answer for who the ricin is for, because most of the same things that make it a poor method of suicide also make it an odd choice for murder at this point in the game. The “advantages” of ricin are in its secrecy, potentially undetectable in both delivery and the death itself until it’s probably too late. Most of Walt’s “problems” are pretty openly hostile to him at this point, so either it’s someone hard to predict because the relationship hasn’t changed yet, or maybe it’s someone in a hospital or something like that—someone who isn’t going anywhere who Walt could get to without a confrontation and slip away from long before anyone realizes what’s happened. We don’t know who that would be yet either, maybe someone from this episode’s shoot-out, but if we’re far enough in the future for Walt to have hair, it’s hard to predict.
The big gun for the Aryan crew is as good a guess as any, whether from remorse or an attempt at tying up loose ends. Hard to imagine Walt going up against them himself even with that much firepower, but they’re still the most logical target for that kind of fight.
Not impossible, but seems like a stretch. Hank and Gomez are still totally off the books at this point. Marie would have to figure out a lot—that they even got a confession, where it is, and then make her case to the police department on her own. I think at least one of the three—Hank, Gomez, and Jesse—need to survive the shoot-out to still have Walt on the run from the law (and it does seem likely from the flash forwards that’s still a concern).
edit: Plus, if Hank’s side “loses” the shoot-out, even if they’re not immediately killed, wherever Walt and the Aryans stand with each other, they’d both be pretty motivated to keep Walt out of jail, and I think Walt could get to and destroy his confession faster than Marie could put things together and find it.
Anyway, other than disagreeing pretty strongly about using the ricin for himself, just sort of brainstorming and talking to myself here too.