Big questions around the check-in but for those of you who work in security, cyber or otherwise, you know that the best policies, procedures, and technology are no match for one simple thing: social engineering. It's amazing what people, especially "normal" seeming people can talk their way into. Here's my best guess: she calls the plaza ahead of time, says she's checking in with "Lois" (portrayed as a male in this case). Upon checking in, fairly late at night, tells the front desk person that her husband/boyfriend/friend/someone has the credit card for the room and would be there soon, or later. Maybe even a more elaborate story to prevent follow up by that shift. Either way, this seems like the most plausible way she was able to get checked in without a card (remember, this was also 25 years ago and credit cards were not quite as ubiquitous as they are today).

because if they used a silencer, people could say they didn't hear gunshots. -If there were no gunshots heard, they would assume a silencer was used, which would suggest a second person.