If staying on Mont St Michel overnight, is it possible to drive back over the causeway to take pictures of the Mont at night or to eat dinner off the Mont?

If you are staying on the mount, you won't be driving on the mount. Unless it has changed in the last couple of years, you will be parking outside the gates. But as I recall, there is an open gate for pedestrians to walk to their vehicles or to walk along the causeway for pics.

I hope you find it as magical as I did.

The entire Mont is pedestrian only. Cars have to be parked outside. Not sure if hotels there have some sort of cart - or if you have to carry your own luggage up.

Taking a pic of the Mont from the causeway depends on your camera equipment and what part you want to show. I think you have to get quite a distance to show the whole thing.

If I remember correctly, those staying on the Mont at one of the hotels are allowed to park in an area closest to the entry, but that this area has metal guards that come up and are locked at night -effectively barring any car passage.

I believe as long as you didn't park in that area, you would be able to come and go as you like in the evening. Just be sure to keep your parking stub in case you need to prove that you paid for parking, but I don't think the parking is "controlled" or paying in the evening. Your hotel will be able to answer this question when you make your reservation.

We've always done our own luggage. That could be answered by your hotel concierge as well in advance. All produce/supplies etc. are brought in on electric carts and maybe some of the pricier hotels have one for luggage.

I'm not sure if others are understanding your question. I get the idea that you are well aware that you park not "ON" the mount, but at the base of it -- at the Mount end of the causeway. And yes, you can drive back or forth across the causeway at night. I'm not sure if you meant "is it possible?" due to tides or other restrictions, but in either case, there is no problem doing that.