Ok, I'm pondering a few things here: (This is bound to be full of spelling and language wonders, please forgive me. I'll correct them if I find them)
1) Is PK a main suspect? My guess is no, not at all. He joined WS just before the Wells 4-part story was released. At that point, after being interviewed by reporters, he would have known the story was about to come out. I don't think a PhD student would join and then commence to say a bunch of stuff that might make him look more guilty, or might conflict with what he told LE. It's a dangerous game to play. Also, any time he said anything it didn't seem like a killer's taunt, so he doesn't seem to be playing a "catch me if you can" game. I'm going with an LE ploy on that one: maybe to release certain facts, or maybe to steer us in another direction. (I was holding off mentioning my feelings on that, but since LE hasn't made progress it's time to move on). I NOW tend to agree with you. He IS a POI but that doesn't make him the killer. DLS was also wrongly arrested for this murder, so I don't want to lay blame where it isn't warrented!
2) Time of death: The part about AG's eyes being wide open, has always bothered me. The way they were described makes me think that death occurred closer to the 30th (time of discovery). The Wells story, and PK, suggest they were normal enough for PK to phone 911 and then go back to check for a pulse. ie: it was mentioned her dark eyes were wide open, almost like they were staring back. No mention of clouding that might be expected after a few days of death. I'm starting to wonder if Jon Wells used that title for one of his articles as a 'hook', to get people to read! Also, PK could have been so freaked out his just said something like 'eyes open' to LE. And, it could be a way to lure out the killer - calling police saying NO, I closed the eyes as an example]
On the other hand, based on what we know about AG's habbits, it doesn't make sense that no comms were made after 6:00 pm-ish on December 27th. My feeling is that this is when the killer arrived. Did he stay until near the 30th, hold AG hostage and kill her then? Possibly, but I doubt it. That's a very risky game, when somebody could drop by at any time (especially over the holiday season). So, I'm going with the time of death being between 6:00 pm on the 27th to noon on the 28th, just because I think she would have normally contacted someone, somehow (email or phone) within those 18 hours. She had been out and about on the 27th, so she was well enough to phone and write more emails. So why did AG's eyes not haze over? Maybe the weather...or maybe Wells and LE mentioned the eyes because that was part of the trophy, or the killer did something to her eyes. (Actually, I thought of that after thinking the SL composite looks like an optometrist I've seen around, and optemetrists are somewhat into "lenses", like photographers...probably all conjecture on my part...but that's how it came to mind).I am thinking that the killer was in AG's house for a few days. I also think the killer sent the "Amazing Grace" emails. I think his final act was killing her.
3) If not PK, then who? The scene has been desribed as horrific by LE, and not so by PK. Who do we believe? Since LE might throw us a curveball to find the killer, did they lead us to believe the scene was worse than it was? I don't think so. If you consider the arrest of DS, LE was looking for someone who they felt might go into a rage and commit a horrific murder. So we can consider it was as LE said.
But who? I think it's definitely someone who had reason to be in the area. If that "drywall saw looking thing" pulled from the ditch is connected, and is in fact a dry wall saw, then quite likely the killer is familiar with how to use one of those, and is possibly in that business. Not to dis drywallers, but someone once said to me, "they're the scurge of the earth". Personally, I've known some great drywallers, but it is a profession that gets a good share of boarderline, semi-transient, alcohol/drug users. I've also seen more than a few like that. So who in that area was into that line of work, and also had reason to be at AG's, or had an interest that gave him motive to go there (ie: cars?)
Also, as LE mentioned about the SV case, is there someone like that (or anyone else) that moved from the area after the crime, and/or began drinking more or using more drugs? (Folks may not have noticed an increase in drinking or drugs because it was the holiday season...when this tends to happen anyway.)
If not a worker, then who? I'm still thinking a neighbor. Quite possibly someone with a history of rage that has flown under the radar. Someone that appears normal, but who has an undocumented history of violence. This type of person may not have appeared very distraught about the murder, but treated it more like "matter of fact". He may not have seemed too vigilant when first told to be by LE.
Whatever the case, I suspect the killer is familiar with dogs, maybe German Shepherds in particular. He may have experience with how to handle them with ease (even aggresive ones). He may even have/had some himself. Maybe he was specifically familiar with AG's dogs from being at her place before, or from the vets.