To be quite blunt and honest I hate customers like this. I mean seriously what do they think a cake is? Duh flour, sugar, eggs, etc.
Now to request some gluten free, ok doable. Sugar-free? Again do-able. But to request a gluten free, sugar-free, egg-free, low-fat, and oh yeah cousin Fred is allergic to peanuts too cake is getting ridiculous.
I have celiac disease and I have to eat gluten free, and I will tell you what no matter what recipe you try. No matter how much you tweak it, it still doesnt taste anything like the real thing, not even close and some recipes are downright disgusting, like this brownie recipe that I made the other day <gag>.
And never mind trying to decorate with sugar-free frosting, much less trying to stack and carve a gluten free cake.
Might I suggest something that you might hate the idea of, but I would refuse this order otherwise. When I have ridiculous requests for cakes like this, I offer to make a cake dummy for asthetic purposes, get yourself a piece of styrofoam, carve away at it. Cover with icing or shortening, and decorate as normal with fondant or BC.
Then make something that will actually taste good that they can eat like a cheesecake or an assortment of mini cheesecakes. A parfait-type of dessert with layers of whipped cream, fresh fruit, and homemade curd or pudding or a bread pudding (which oddly enough tastes pretty good made with gluten free bread). If you pm me I can give you help/tips on making these.
Truthfully, as a person who cant eat most cakes/brownies, I just either excuse myself from eating what the rest of them do or I try a tiny piece and pay the price later. I would rather eat a tiny piece of something that tastes great, rather than a huge piece of something that tastes like crap.