Given that massive stack of money I don't see Walt having to resume cooking for any reason. And they've done the 'Walt goes back because he realizes he really wants to' thing so many times in the series it would be lame if they did it again.
I don't see the series as having any moral message. I see it as more of a character study of Walt. Here we have this man who was always brilliant, who always had the capability to do amazing things with science, but he always played it safe and never took risks. Then he realizes he's going to die and suddenly unleashes himself just out of spite and anger that he spent fifty years of his life holding himself back. Walt always wanted to be a MAN, Sergio Leone movie style. He wants to be the mythology. His name is fuckin' HEISENBERG.
If the show was trying to let you know crime doesn't pay, it's miserably failed. Look at that nine or ten figure pile of cash. Look at all the money Fring was making, and would have kept making it if he'd just listened to his instincts and not done business with Walt. The show tells you that it's impossible to deal drugs without having to deal with the violence, which is true. But that's not a moral message, it's a real world response to Walt's conceits about being a criminal.
If there is a core message of the show beyond being a character study, it's that moral lines are easier to cross than we think and even easier the second time. Once you 'break bad', it's broken.
I think you nailed it. And this being the case, I think it frees Vince and his staff to do more with respect to Walt's end than if they had to make some moral statement. BB was never quite as artfully presented as The Sopranos or Mad Men, but it has always had great contradictory and complex characters and has been full of surprises.
So I don't think we'll see Walt go down in a hail bullets, but suffer some kind of Twilight Zoney ending where he gets some of what he ultimately wants but have to live with an unberable cost.
The DEA may be aware of the German companies' involvment in trafficking but they don't have the "goods" on them. Perhaps that will be something Walt offers to provide.