I hate to rain on anyone's parade. I watched the Tena Bar video on You Tube, its just too far fetched for me to believe Cooper would ever have returned there to plant $5k in in the sand? Why? If he wanted to prove he made it, he could have put some of the money in circulation immediately. He did not want to get caught obviously. I still believe he made it. I wonder if he knew the money was likely recorded early on? Like when he first laid eyes on it? Would it have been consecutive numbers or would they have scrambled them? And it is strange the money was in the bands and did not come apart.
The letters are the other mystery within a mystery. The code and Rackstraw? Well they sure do not believe he was DB, so I don't either
For what it's worth, I have never believed that DB planted the money to fool anyone. But it was a hell of a lot of cash to walk into civilization with eventually without a knapsack. He could not possibly have remained inconspicuous, so unless he had an accomplice or a way to track a vehicle he'd left somewhere (Mannix has informed me such devices did exist, lol, though I'm not sure how practical they'd be to jump with) he'd have to stash the money for a time until the area had cooled off a bit and he could come back for it. (McCoy hid his in a culvert but they caught him before he could retrieve it.)
Some of the money - the bills found - may have been damaged before he could retrieve them, or merely fallen loose from his makeshift money belt while he was still in the air, ending up at Tena Bar god knows how. It's never left my mind, though, that the bills didn't fan as if they'd been in the water there. However they got to Tena Bar, I do not believe they washed up, and the loss of money, which was attached in a makeshift way to Cooper, does not immediately indicate the loss of his life. He may have only gotten away with the crime, not the money. But I think if he'd gotten his knapsack, the money would never have been found at Tina/Tena Bar.
I don't know what to make of the letters but I don't really believe they came from DBC. If the cops were assuming or at least putting the theory around he was likely dead, who would risk arrest stating otherwise? There's nothing in his behaviour on the plane to indicate he was mentally disturbed enough to do that. It would be a stupid thing to do, and he didn't do one stupid thing on that plane.