Were they planning to make Marion one of the Yin Yang killers? *spoiler*

You mean Mary? I don't recall a Marion

I never got the feeling they changed it at the last moment or anything. To make Mary one of the killers would've been too obvious, as they pointed out themselves as Shawn was going through Mary's suspicious ways. The look he gave Yang while they visited her in lock up, I believe, was just him being giddy about having an up and close moment with a person and case he's been obsessed with and has dedicated his life to. He was, quite simply, just fanboying around haha...because it's Mary, and that's how the character is. I don't think there's too much more to read into it than that. I think Yang gave in at that moment because he was onto to her and he was smarter than she thought or that was the moment she realized she could maybe get back at her dad or come out on the "better" side of this thing. Either way, I never took it as a confirmation of his involvement, but another bread crumb to maybe make him stick out more. Mary was a red herring basically, until his death.

I guess we'll never know if they ever intended anything else unless they say so. They don't seem like the bunch to conceal a bit of information like that though, especially since it's been years since the story played out anyways. I don't think it was anything like that, but who knows!

Well. I feel vindicated. There's an episode in season 6 where the guys team up with a bookstore owner who was big into true crime stories. He brings up the Yin Yang case. Guy remarks "Am I the only one who thought Marion was the killer?". I took that as a little acknowledgement to the fans who were less than pleased all the hints about Marion being involved were ignored. So at least it wasn't just me.

It never felt like they changed the storyline to me. They definitely made Mary look guilty on purpose. But I always felt it was supposed to be an obvious red herring. If Mary was intended to be Yin at the end of the second episode, then there should have been some clue when Shawn saw him at the pier, like the ankle weights. But there wasn't any indication that Yin seemed familiar to Shawn.

While I certainly don't argue that Mary was an obvious likely suspect, it just doesn't feel that they changed directions. More that Mary was always going to be a red herring, and sometimes the obsessive creepy guy is just an obsessive creepy guy and not a murderer.

Unfortunately, I don't remember that exchange between Mary and Yang. But Mary has always been obsessed with Yang, and Yang's crazy, so that look could be anything.

Also, his name is Mary. It's not just Shawn who calls him that and everybody calls him that but his real name is Mary. He is credited as Mary.