Absolutely! In fact, I find meringue buttercream much smoother and bubble free after an overnight rest. I regard meringue buttercream as temporarily shelf stable because its so high in sugar and fat, but for people who are concerned, just chill it and then take it out an hour or so before you want to use it. If it breaks when you rewhip it, just keep beating.

Yes! You will need to re-whip once you are ready to use it.