I still fail to see a connection to Emerich fingering Peterson. this would imply he had two people involved while suspecting who he thought was Cooper. he states he thinks he knows who it was and it's appears to be Whitney vs Peterson.
Now that I've read further, he does mention the word accomplice as a possibility but doesn't explore it. He mentions the name Whitney as prime suspect. But there's no date on those papers, so I don't know if this is an earlier theory or a later one. He could have thought Peterson at first but someone else later, vice versa, or never had SP on the radar for it at all; if he did think SP was in on it, the only conceivable reason he wouldn't mention it is that SP was still alive and he had no proof - we don't know how long this Whitney lived or if Emrich was aware of his alive-or-dead status. From this, however, there is nothing to link to SP.
Personally, I've never been into accomplice theories, though there's nothing to rule them out; however, this Willard Whitney is a question mark entirely. Anyone I can find with the name online is either a little outside the age range or has no apparent connection to the region, and nothing re skydiving. I figured as expert skydivers are a narrower group, the name might pop up among older skydivers but I'm finding nada; Whitney could have been a frequent visiting amateur to Issaquah but not an expert, maybe? There's a famous journalist with the same name but he seems too young, and a famous racing driver but he doesn't appear to have any connection to skydiving or any need for money. A deeper dig is needed as a lot of people have a variation on that name. But Emrich didn't pull the name out of a hat; he existed. Whitney is at minimum a new suspect and a wholly unknown quantity at this point - NO old photos online of anyone with that name. Emrich said Whitney was a "lookalike skydiver" to the Cooper sketch, but I'm unsure which sketch version.
The other thing is, SP said he knew Emrich - not the other way around. To my knowledge so far, Emrich never mentioned SP by name, but he apparently kept lots of stuff on the case and this is only a fragment. I'm not ruling out aliases; however, if SP had been operating under an alias at Issaquah, I don't think that he would even have mentioned the place online at all. Any other stories SP has told about his skydiving online have, as far as I know, been verified, and he was a known figure among skydivers.
Of course, at the end of the day, Emrich THINKING someone could be Cooper doesn't make it so, but it is interesting he focused on this Whitney guy and again, I REALLY would like to see the members lists of that skydiving club. (It's also possible Whitney was outside any club but was a frequent flying loner Emrich remembered.) I hope we will learn more.