RE: Stabilized whipped cream or Buttercream?

1. This would depend upon the regulations in your area. A whipped cream cake would need to be refrigerated.
2. It would be fine to fill with whipped cream and frost with buttercream, but again, the cake would require refrigeration.
3. To prevent the fruit from soaking into the cake, put a thin layer of buttercream between the cake and the fruit filling.