Even more strange: why would someone plan to stay the night and not bring sleepover clothes when they had to work the next day and didn't bring work clothes either??
Christina could have had overnight clothes and work clothes in her backpack. I can't see her changing into her pyjamas when there were still all those people there, but borrowing a t-shirt to make herself more comfortable, makes sense to me.
She probably had the longest drive home of everyone there, so it would make sense that she would be the one to stay the night, but it seems as if the no-one was in a rush to leave, so she had no place to sleep.
SB must have been parked near the apt (as they drove from the burger place in her vehicle), but it sounds like she was getting unwelcome attention from EA. If she had left to drive home, while EA was still there, he would surely have asked her for a lift to the garage, where he was parked. SB may have thought that once he was in her car, she would never get rid of him, so decided to wait until he left.
Of course, I don't know if this is how things went down, but it's a possibility.