They came in plastic tray. Why couldn't a tray have been carried up the stairs?
There were bullets emptied out in the kitchen onto the surface. Of course, I don't know enough about guns to know how bullets are packed.
If you're right, yes, she could have carried them upstairs with her. What was Nevill doing whilst she was reloading in the twin's bedroom though? Still sleeping? He wasn't asleep when he was shot in the bedroom IMO.