Your Royal Icing recipe does include cream of tartar and lots of confectioners' sugar, doesn't it? If so, try adding more confectioners' sugar and using lots of mortar. The only time I assembled a gingerbread house was at the Coy's many years ago. I thought we had to let the mortar dry overnight, but all the recipes I can find say that the frosting should dry in 15 to 20 minutes. Good luck!

Oh, BTW, if Lis doesn't get to eat the house within a couple of days, it's likely to have acquired authentic brick-like qualities. Eating it in January or February could be difficult.