Just want to say that you’ve picked one of my very favorite cakes from RHC, I think it will be absolutely delicious!
My suggestions would be:
-Freeze the cake. This will allow you to make it as far ahead as you like and then relax and enjoy the trip and wedding (or at least to worry about other things). Even if you make it on the Wednesday, I would still freeze the cake, because the lemon juice will gradually loose its tanginess the longer it’s stored in the fridge. You could opt to keep the cake frozen during travel by using a cooler with either dry ice or regular ice, and the hotel may agree to store the cake in their freezer for a few days (be sure to keep it completely wrapped and boxed so it doesn’t absorb odors- they may put it next to the fish, and to label it so no one is tempted to open it to see what it is). If you opt to allow it to defrost at the hotel, then keep it in the fridge in your room and it will probably be just fine. Perhaps not as tangy as if you had frozen it, but this cake is a particularly good keeper- it will still be great.
-Make the frosting first. You can make the lemon curd a day or more ahead of making the mousseline, and you can make the mousseline a day or more ahead of making the cake. Just be sure to leave plenty of time (hours and hours) for it to fully come to room temp, then re-beat it just before frosting the cake. If storing it, be sure it is totally sealed against fridge/freezer odors and any residual odors on plastic tupperware-type containers.
-Frost the cake as soon as it is syruped and completely cool, to seal in freshness, flavor and moisture. Many find it easiest, when trying to achieve a smooth surface and opaque coverage, to crumb coat and chill first, then frost and decorate. You can finish the cake completely, including piping, etc. before freezing. Freeze it uncovered long enough for the frosting to set hard, then wrap in two layers of plastic and one of foil.
-If the weather is hot and the wedding is outside, keep the cake cold until you need to set it out for the reception. Display it in the shade. Allow at least three hours in warm weather for it to come to room temp from the refrigerator, longer from the freezer.