I want to make ahead champagne cupcakes & champagne frosting (so I can use the same bottle of champagne), but I'm not sure if the frosting will keep several days. Will anything funky happen to the texture or taste if it's not eaten immediately (consists of butter, sugar, & champagne)? Or will it keep like a regular buttercream? Thanks!

The champagne will evaporate, thus probabley won't be as strong in taste, but that's about the only thing that will happen.

sometimes, the champagne will weep out of the buttercream. I usually use a champagne simple syrup and then also a dram of the champagne flavoring.

Reduce the champagne to 1/4 and use in IMBC. No need for fake ingredients. It will keep for two weeks covered.