When I have family come to visit, if they are independent types, I always give them keys to the house. That way they can come and go w/out being concerned if someone will be there when they return.
How would AS know if and when RN would be there? Why would he make his brother dependent upon his girlfriend to get in and out of the home?
I do not think she left the doors unlocked that night when she was showering and such. But if she decided to kill herself, she may have unlocked the door on purpose. She would want to be found in that case.
JS was at the hospital with Max when the three of them went to dinner. Do you really think he carried around an extra set of keys to hand out to AS? I think if it was the case that he had keys, that Rebecca would have given them to him, but when?
She picked him up at the airport, brought him to dinner with JS also, then they went home and AS said he went to the guest house, she to the main house. I would think he would have mentioned being given keys and by whom.
As for unlocking the door if she committed suicide, why? She was easily seen in the courtyard. AS would not need to enter the residence to find her.