If you look hard, you can always find a reason to fire the PM. The key here was whether the competition was close - would Matchstick have won if the sofas had arrived? The editing made that unclear.
On one hand, they showed the Westin people clearly impressed with the Primarius room and wondering about the theme of the Matchstick room. On the other hand, several people referred to the competition as close. I don't think we know the answer.
Dawn's firing makes a lot of sense if it were a close competition. She has clearly been a drag on the others - refusing to paint is just not an option when you have a group that needs to stay up all night, regardless of the reason why.
The problem with this show concept is twofold:
1) The casting is based on Mark Burnett's theory that reality television works best when you have a group of outgoing, type-A people who aren't necessarily the best suited for the task (i.e. business).
2) The contestants have long since learned that the PM will be fired as often as not. Therefore, there's very little effort to figure out who really did contribute least to the task. What made the Trump show so good in the first season was that they hadn't figured that out yet, and the boardrooms were far more revealing.