I'm not convinced the murder happened in the guest house.
The black paint on the door, the carpet, the plastic bag, and Rebecca's nipples and buttocks, puts her in the balcony room.
There are two black paint marks on the carpet and I'm wondering if this was transfer paint from her nipples if she was laid there and tied up. The plastic bag would be roughly where her head would be in proximity to those paint marks.
I agree with you, I also believe a significant amount of time passed with AS and RZ in this bedroom. However, I believe RZ was initially somehow incapacitated, rendered unconcious and sexually assaulted in this room. Maybe she was hit, maybe she was suffocated with the bag. I tend to agree, it was likely the bag, (due to the probability of auto erotic asphyxiation videos being in the perpetrators video library, from what we have heard)
I also agree the painting was likely done whilst RZ was unconscious in the upper room, which is why there is no evidence of injury ocurring during the sexual assault. There was no struggle.
The paint on the body is likely transfer from the gloves that were worn IMO. Acrylic paint dries relatively quickly, so it would have to have been transferred within a relatively short space of time, while she was still in the room.
I believe that RZ likely regained consciousness, so she was then forced to the guest house. She may have cried out up in the bedroom which was the catalyst of why she was taken somewhere more secure, or she may have called out whilst being forced outside walking across the courtyard.
I believe the T-shirt gag indicates a lack of full planning, so using available items from the room at that time - perhaps she was bound initially with the tape in the bedroom on her legs, perhaps a T-shirt was used to bind her hands?
I imagine that it was the huge knife found with RZs prints that was used to threaten her and coerce her into doing what she was told.
IMO the rope binding and hanging took place in the guest house storage room, she was hog tied and left for a time, maybe this was when the music that was heard was put on?
This was when she was alone, she managed access to the knife to try and cut herself free, she was sitting on her feet ( soles up) and this accounts for the blood found under her big toes. No doubt the perpetrator thought being hog tied meant that she couldn’t escape, he was sloppy and left the knife. RZ was industrious, agile, and fought for her life to free herself. This is truly the hardest part to consider.
So, in summary I believe there was activity in the bedroom, but RZ was mainly incapacitated, and she was then transferred to the guest house. Eventually her dead body was carried from the guesthouse and lain on the grass, while the rope and bed were staged, and the wipe down completed.
Edited to add.
Hearing about the sort of individual RZ was, no doubt she was frightened, terrified in fact, but I can imagine she would be the type of person who would try to reason with the perpetrator, she would attempt to convince them to stop by persuasion. I believe she probably tried hard to reason with her attacker....
Just my opinion....