Two Crucial Plot Holes ***Spoilers***
1. When Ransom came again to the house, during the funeral, to switch back the labels as it originally was, he should have realized that the bottle with Morphine label is empty I.e. the bottle with Morphine content was full and the bottle with actual medicine was empty.
He should have realised then and there that Marta gave the actual medicines and not the Morphine.
2. How could Ransom kill Fran ? He was in the police car and getting interrogated.
Ransom and Marta were caught after the car chase. While Marta was accompanied by Detective Blanc in her car, Ransom was taken into the other police car with Lt. Elliot. They drove and started together. While on their way, at some point, Marta requested Blanc's permission to pull over at the place of meeting, telling that she wanted to collect some stuff.
How could Ransom be there and that too almost ten minutes before Marta, so that he could inject Fran with Morphine ???
Later, Detective Blanc tells Marta that Ransom spoke out the whole story and that Marta should now tell it all to him (Blanc) in detail during their enroute to the Thrombey's Mansion. So obviously, Ransom was with the Police custody and not loose enough to go and kill Fran before Marta's arrival at the meeting point.
I think, these two holes almost kills the movie.