I don't believe Cooper himself wrote those letters - if all he wanted was to get away with $200,000, better to let the feds think him dead. More likely a random, mentally ill person wrote them, and moreover (as several letters do look like they have a common source) that person was nomadic. TK being or even meeting Cooper seems like a long shot, but I could totally see him admiring Cooper's "beating the system" and writing the letters to torment the cops.
This is a good point. We've seen posts elsewhere from "someone" who believes Cooper buried the money at Tina Bar after the John Doe charge in 1976 so the FBI would think he was dead. That would be absolutely the opposite Cooper would be expected to do. By that time there was probably only one agent assigned to the case, and only dealing with evidence as it came in. Dropping money on the beach to be found would stir up a whole bunch of new activity with several agents and attract lots more attention. Not smart.
Tena Bar, the most I make of it is the money got there somehow, wound up in the condition it did somehow, and nothing else wound up there. Cooper burying it
briefly after the jump and leaving some behind by accident - may be possible, and Tina Bar has the advantage of being a marked spot in the area, not just a random clump of trees in the woods that would be hard to find again later; McCoy did hide his loot in a culvert for intended later retrieval. The more I think about it, the weirder it is that of all the places the money could have wound up in that wilderness and along that route, it wound up in a place that is publicly marked, easy to find. However, Cooper planting some years later to appear dead when they pretty much already thought he
was dead and, as you say, had other fish to fry, would be poking a hornet's nest for no reason. That would be among the LEAST likely explanations, logically, of how the money wound up at TB. We don't know how the guy thought, really, so none of the explanations are impossible, right up to "Cooper taunted bear with loot, bear ate Cooper, used bills as toilet paper, some of it stuck to his butt and only dropped off at Tina Bar." Any stands I take on this case are not at TB, lol.