.....If the time of the murder can be narrowed down to after 6 pm the night of the 27th and before noon on the 28th (we might expect she would normally have sent an email or phoned someone by that time) then maybe there are some things to consider about possible suspects.
The 27th
If the perp called after dark, I think it would have been someone AG really trusted. Someone she would've expected to see drop by, if out of the blue, at that time of day: a friend or neighbor. And equally, the perp would probably have expected to be let into the house or garage at that time. Anyone else at night, and I think the dogs would have stayed out of the basement. (assuming the guy from dotr's link, in posting #357, is right about the basement part)
The other possibility for after dark is that she bought/forgot something when out earlier that day, and she opened the door for someone dropping it off (perhaps vet meds that were out of stock til later in the day, or something she forgot somewhere).
The 28th
If after the start of normal business hours on the 28th, then she might have opened the door for a trade-person, or someone else, in addition to friends and neighbors. If not a friend or neighbor, she probably would have recognized him; she had dealt with him before somehow and trusted him to some extent.
(We never did learn who it was that said they heard from AG, and that she wouldn't be coming to coffee on the Wednesday. Was that the "friend" who stopped by? Or was the report just a media mistake? My guess; a media error. But maybe not. Knowing for sure would be helpful.)
Since AG was a night person, would she normally have sent more emails after 6pm on the 27th? If so, then I'm thinking the 27th is probably when the murder happened, and at a time that wouldn't be considered too late to drop by. Or at least the perp arrived at this time. If so, he was probably a friend or neighbor.