Having read the article at the link, and further links, I agree the arguments are compelling.
But thorough as they seem to be, the articles fail to address a few very relevant points:
Was it possible in 1972 to track the approximate location of the 727 when Cooper jumped? How about the on board navigation equipment? Did keeping the plane at 10,000 prevent any attempt at tracking?
Next, someone had to prepare the parachutes that were delivered to Cooper and would know for a fact whether the parachutes were fully functional or rigged to fail.
The FBI would certainly know that and this long after the fact, there is no reason to keep this detail secret.
What is the reason he asked for two of each type of parachute? Maybe he wanted to quickly inspect one without having to rerig it. Also there is the possiblity (from the point of view of the ransom collectors) that an innocent person would have had to use one chute and thus it could not be rendered inoperable.
Only approximately. They don't know exactly when he jumped, since he sent the one flight attendant remaining on the plane up to the cockpit as soon as they took off.