Can I make this cake portion a day in advance and refrigerate overnight before icing? Also, should I keep in the fridge before serving for an even...

Yes to the refrigerating overnight for the cake alone. But if you are frosting just before serving, there is no need -- in fact, don't - refrigerate it again before serving. Cake is rarely at its best served cold. It should be at room temp.

yes to both (bc of buttercream). eat it cold or let it come to room temp, according to taste