What was an accident? The disappearance of DB Cooper? No. It's safe to say it wasn't an accident.
No. It's also safe to say that Cooper didn't attempt to draw attention to himself. At least, not after he jumped.
They still are. But, then, no one knows who Cooper really was, or what he really looked like.
No. Like our legendary furry pal, Cooper didn't want to be found. That's the whole point. He didn't want to be found, he just passed through, no one was even sure what he looked like, but there was still evidence that he was there. If a crafty human being intent on avoiding detection couldn't slip through the PNW without leaving evidence behind, why would you think an animal could manage it for decades?
I think the Bigfoot community would beg to differ. Not to mention the '10000's of loggers, rangers, etc." that you yourself claim are "combing the wilderness, checking under every leaf, every rock, etc.' looking for any sign of even one creature.
You just said a few posts ago that "It is not unlikely for a single one to evade detection." It is either likely or unlikely. It can't be both.
Is it possible that a single one could escape detection over, say, decades? Anything is possible. But likely? No. Even in the vastness of the PNW, people hunt, hike, fish, camp, etc. That increases the likelihood that somebody, at some point, would have found some verifiable evidence of an unknown creature, even if they didn't spot the creature itself.