Question: I've probably missed something, but at one point Michael rescues Sarah from the very room that they are trying to access for the escape - why didn't they use that route to get there?
Answer: She wasn't in the infirmary, she was actually in the sick bay.
Answer: Because the path from underneath the Doc's room had a new pipe replaced. The pipe was too big and firm to tear off so they had to change their plans last minute.
Question: To create a lock-down situation in the prison, Michael breaks the AC unit and the doctor later says that it's the hottest April on record. However the beginning of the episode where Lincoln sits in the electric chair and is then let out again (forgot the name of it) shows it snowing pretty heavily. Up to this point the programme has been set within one month so my question is this. Is this meteorologically possible? And if so is it probable to have heavy snowfall in the hottest April on record?
Chosen answer: While this might be an oversight in the show itself, speaking from experience there is a possibility of snow coming and going despite being a warm month. I remember in 2014 going from a high of 8° Celsius (46F) and sunny to -16 C (3.2F) and heavy snow the very next day. The statement as you put means it's the hottest April on record, not the hottest temperatures for the entire year. It may have been unusual temperatures for that time of year and warmer than usual. The snow might have only been for a brief period and disappeared shortly after.
Answer: She wasn't in the infirmary, she was in Sick Bay, a different location in B Wing.