As far as Pak knowing Hackett, I derived that from how Hackett inserted himself in the case by calling Mari, Shannan's mother.
When a wrongful death suit was launched against Hackett, it stated Hackett rendered care to Shannan, and this was corroborated, by what Hacket said to Mari. No matter what kind of lawyer John Ray is, he is not going to the expense and time to initiate a lawsuit unless there is some sort of chance of some kind of win.
We know that Pak and Alex Diaz when they went searching at Oak Beach a day or two after Shannan disappeared, they met with Hackett. I find that unusual the only person they had contact with, that is reported anywhere in the media is Hackett. It makes sense if Pak and Hackett knew each other that Pak would visit with Hackett.
Finally we know that last place both Pak and Shannan were seen was on the road to Hackett's house. So it would be easy for both Pak, Shannan and Hackett to meet that night. Hackett allegedly told Mari he treated Shannan.
Regarding people at Brewer's house when Shannan was present. There has been speculation over the years that some members of Suffolk County Law Enforcement might have been present. If some additional men were present it could explain some missing details. Details like was the presence of these additional men the source of Shannan's fear?
If there were additional men there were they associated for criminal purposes?