I would say, as it is a special occasion, to leave your mash in the freezer for another time, (perhaps as a topping for shepherds pie between Christmas and New year when all you really want one night is some comfort food) and start afresh. I wouldn't add cream for Duchesse potatoes and only a tiny bit of butter because in my experience this can make the mash 'glue-ey' although I like to season well with pepper, salt and a pinch of nutmeg.. Egg binds it of course and the brushed yolk makes the outside the crispy shell which is what you want.
This is only what I would do of course.
Life is like good wine.......best taken with friends x