The purist answer is that coconut-walnut IS german chocolate cake... the practical answer is your client is paying and there are honourable swaps 😄
The classic build alternates dark-milk ganache with the cooked coconut-walnut custard filling. For your no-coconut version, two fillings hold up structurally between those layers: vanilla pastry cream (thick-cooked, it has the body to carry the stacking) or simply a thicker layer of the same ganache - a double-chocolate german, no one has ever complained. Keep the walnuts for texture in either, toasted.
And a middle path worth offering: swap walnuts for PECANS - that's actually the traditional American version - with the same cooked-custard method minus coconut. It keeps the caramelised-nut-custard soul of the cake without the offending ingredient.
Call it "german chocolate, house version" on the box and everyone keeps their honour intact 🙂
