Sharing dessert?

In my experience, yes.

Do what you want with it, your the one paying for it.

Nope.
I also often take plastic cartons with me to restaurants and save our leftovers (we are trying to avoid over-eating). Never had an issue (although didn't do it at La Gavroche).

This works very well for us as you can stop when you're full and don't feel like the food is wasted and also provides a meal for the next day.

It has really helped me maintain my weight when eating out many times a week.