I've said it before and I'll say it again: We have everything we need right now to solve the case. We have enough pieces of the puzzle to crack the riddle. We are simply faced with a mental challenge requiring a Sherlock Holmes approach.
I believe the key to solving the DB Cooper riddle involves putting yourself into DB Cooper's mind, then seeing what evolves. Here's an example:
1) We know DB Cooper left his tie and alligator tie clip on the jet. In all likelihood this was done accidentally, after all, the guy was anal about collecting all evidence.
OK, with that in mind, how do you think DBC responded when he realized he left the tie and tie clip behind?
I assume he was very concerned that the authorities may be able to garner a fingerprint from the tie or tie clip or in some other manner figure out who the owner of the tie and tie clip is.
In my mind, DBC probably decided to lay low for quite awhile after the skyjacking, in part, because of this. In other words, I highly doubt he spent any of the money and I highly doubt that he had it hiding in his closet or anywhere near him, just in case the authorities came knocking at his door.
2) I'd being willing to wager that DB Cooper read everything he could about the skyjack in the newspapers immediately after the event. After all, he would want to know what the authorities were thinking, where they were searching, etc.
OK, so what if you're Sheridan Peterson and you're DB Cooper and you read the next day in a United Press International article that the authorities are looking for a former smokejumper? Additionally, what if you're Sheridan Peterson and you're DB Cooper and you read that Linn Emerick--a guy that you know personally from skydiving at Issaquah Sky Sports years earlier--states that he thinks he knows who DBC is based upon the sketch and that this person is someone who used to skydive at Issaquah Sky Sports?
May I suggest that in this case Sheridan--if he actually was DBC--was probably petrified and very concerned that the authorities were hot on his trail. Again, with this in mind you would think the Sheridan would distance himself from the money, and frankly the country, for quite awhile. In other words, his thoughts and actions would be shaped by what he considers a threat.
Where all of this leads and the story it tells is all part of solving the riddle. There are plenty such examples in this case and plenty such paths that we can take as we attempt to figure out what happened and why. Remember, a mind is a beautiful thing to waste.