We have a flight to Nantucket from Newark with a 40min layover-is that enough time to switch planes?

As long as you walk hastily to the other gate, you should be ok.

It should be but you might want to check which terminal you're arriving at versus where you're leaving from. If you have to switch terminals it's going to be really tight.

Yes, but need to be walking fast, you should be ok, with less crowd these days.

Nowadays yes