We've heard this framework of next day events, but I agree we don't necessarily know the order of how things proceeded or how they come to happen. For instance, we know he contacted HT and there was this arrangement of key pickup, but as discussed it's unknown on if he had attempted to go to SW first already or not. Seems just as easy that he got ahold of HT to find out if she had seen LS that morning, and when she said she hadn't, they arranged the key hand-off so he could go check right then. Not that it couldn't be done, but remember there is key card access required at SW and the elevator just to get to her apt door.
How would a guest visit at SW? Can you buzz the apartment of the person you are attempting to visit?
As for the jilted bf scenario where he goes looking for her, wouldn't this trigger a cell phone ping discrepancy in his story? Leaving his cell phone behind if he was desperately seeking her out would require some serious pre-meditation.
Hypothetically...
Or simply forgot it and left it on the table. Or was only intending to go a few blocks and intentionally left it. I don't think premeditation is necessarily required. But if he did leave it behind, and then snapped in a confrontation, he's probably not going to go back to retrieve the phone at that point. Not only would premeditation not be required, but he wouldn't even have to have knowledge that the phone could make him traceable. It would just be a lucky break for a criminal in that scenario.