This is what I'm saying:
RS and AK were at the cottage the day of the murder up until 4pm.
They were the last to leave before MK returned home.
RS and AK love hugging and smooching. Maybe they weren't paying attention and didn't latch the door properly at 4pm when they left because they were busy hugging and smooching.
The door swings open and stays that way until RG happens along sometime before 8:53pm. He sees the open door. He sees it as he chance to burgle. He goes in there and CLOSES the door and latches it.
MK happens home, opens the door and latches it. She goes in her room. RG is on the toilet and hears her. He attacks her.
AK returns home, sees the door open and thinks to herself "Oh, my god, I left this door open all friggin night." Or something like that. Then she sees what's happened and stages the break in, because she feels guilty that she was the one who left the door latched unproperly at 4pm the previous day.
Now I don't know if that's possible unless it's true that she then found MK's keys and locked the murder room to distance herself from MK as she staged the breakin.
Does that make more sense?
**for those who don't know, I'm completely guessing up another scenerio.
Oh, of course. Yes, it makes sense and I should have figured it out. The point being that (hypothetically) RG entered due to AK's negligence BEFORE MK returned home. Yes, of course, AK would feel guilt for that. (I don't see her staging a break-in over it, but I can see the guilt.) I'm sorry you had to explain it twice.