Ending A is the only one that makes sense *SPOILERS*

Ending A has its holes too:

How did Yvette know that Mrs. Peacock would be standing by the door and drop the knife there? And the exact moment when to open the door to grab it without anyone noticing. Or even that Mrs. Peacock would start screaming at all. Then she not only sneaks up on and kills the cook, but then picks up her lifeless corpse and drags it into the freezer, and returns back to the billiard room to start screaming in less than a minute. All 3 endings have the plot hole of 1 woman dragging the dead cook into the freezer, when later on we see 3 women and a man have a hard time moving her.

Likewise, she has to not only sneak up on Mr. Boddy in the hall, but kill him without anyone hearing anything, and drag him to the bathroom, hide the candlestick on top the door frame, and rejoin the group in time to help carry the cook back to the study.

They never explain how Scarlet got the key to the weapons. Then after she and Mustard separated in the ball room, she has to go all the way back to the study, which if you use the board game as a floor plan (the movie does stay true to that) means she goes from the very back of the house to the front. There she destroys all the evidence and unlocks the weapons, also why get the wrench when you could just use the candlestick that was already out and had already been used? Then she goes to the conservatory (back of the house again) and goes thru the secret passage to the lounge (front of the house), sneaks up the motorist who she didn't know would have his back to the fireplace that just opened, and kills him. Then she has to return to conservatory (back of the house) and rejoins Mustard. And what was Mustard doing this whole time??? Either his partner was killed in the ball room, or she's taking a long time to look at nothing. And he doesn't see her move back and forth across the main floor. And if you don't count the hall or bathroom, there are 8 rooms, with stranger locked in the lounge. So that leaves 7 rooms for them to check. He left her in the ball room, and there isn't much to check in the dining room, billiard room (which they already checked) study (why they spent most of the evening), or the library. And they went to the conservatory once they reunited. Was he just sitting in the kitchen the whole time??

Then for the purge, she has to separate from Mustard again, get to the power, get the rope and lead pipe, hope Yvette comes down stairs and bait her to the billiard room to strangle her, without Mustard hearing or walking by. Likewise she has to get into the library without the cop noticing the door opening, or Mustard walking by. And when the doorbell rings, she goes out to hall, gets the gun and shots the singing tell-a-gram without Mustard, or anyone else coming to the door. Then she leaves the body there on the front doorstep, knowing that the cops are arriving any minute.

Finally, the biggest flaw of all; Scarlet wouldn't have committed murder because she wouldn't have paid the blackmail in the first place. She would have had multiple cops on her payroll, or guys would come in for free samples in exchange for her protection. If this cop tried to inform on her, the others would help her. And when Mr. Boddy tried to blackmail her, she would laugh in his face. Just like Mr. Green said, the whole time would be implicated if she were exposed. Even if Boddy turned her in and she was arrested, she makes one phone call, and that police station is flooded with calls from politicians, and maybe even higher ups in the police force demanding that the charges be dropped.