If this is the case, OP would have about a 13 mile cab ride after he returns the rental car. This means he would probably be running quite a difference in price between car and train.

(Fly into MXP around 7am and head straight to Sarzana where we have friends who will show us around by car. Then onward to Lucca, Florence and back to Milan (where we will fly out of). I do not want to rent a car as I would rather take the train but that 5.5 - 6 hr train to Sarzana is going to be brutal after a 8 - 9 hr flight.

Once we leave Sarzana, the train is definitely the way to go (although a bit afraid about getting around Lucca as I've heard a car is the way to do it).

I'm not sure you need a car in Lucca. It is a walled city, and most people stay within the walls or walk around on top of the walls. You have to park a car outside the walls. You only need a car if you want to explore around the area, not to visit Lucca.