When did unaided intruder get in house?

[camgarsky4]

If neither Lizzie, Morse or Bridget were the killer or an accomplice in the killing of Abby, when did the killer come into the house and make their way to the guest room.

Backdrop:
Front door was locked from 9pm Wed night until 10:40ish Thursday.
Cellar exterior door had been locked since laundry day earlier in the week and until police arrived Thursday.
Side door was locked from 10pm Wed night until Andrew went to throw out his slops by the barn around 6:30am.
Morse, Andrew, Abby and Bridget were in the sitting room, dining room or kitchen from 6:00 until 9am when Andrew left and Bridget was throwing up.
Lizzie was in kitchen 8:55.
Bridget back in kitchen around 9:05. Lizzie not present, so presumably in her room on 2nd floor.
Bridget recalls talking to Abby in the dining room slightly after coming back in house from vomiting. 9:10ish
Abby makes her way back up to guest room to put on the shams. She would not have entered the room if a stranger was already in the room.
So no one in the guest room, Lizzie was in her bedroom and the upstairs clothes closet was kept locked. So intruder could not have been waiting for Abby.
In theory, someone could have snuck in when Bridget was in the cellar getting a pail and brush. But, once in the house, how did they know to immediately sprint quietly up to the guest room to perform murder carrying a hatchet? Keep in mind that Lizzie must have been in her bedroom next to guest room.

I'm not a statistician, but the odds must be astronomical for an unaided intruder to have killed Abby.

I'm sure I'm overlooking a solution. Can anyone suggest a timeline for an unaided intruder to have killed Abby?