I've never had much luck reheating mashed potatoes in an oven, especially once piped. If you want to do them in advance, have you thought about doing duchess style potatoes, which are basically mashed potatoes with egg yolks added. They hold together better and aquire a nice golden tone too. It shouldn't be a problem adding the chives ad horseradish too. Good Luck!
5/27/12 at 6:19pm