For example, how would one say 'More Cuba Libres' in Spanish?
I mostly hear Cuba libres, my view point is that of a native speaker, not a linguist, so it may not be right, but in matters of languages, how the living language is, is sometimes more important that what books say.
One clear example is Medialunas ( literally half-moon, means croissant ) although sometimes but not often, I've heard medias lunas
The problem here is the difference between proper and common Spanish. In proper Mexican Spanish a proper name is never pluralized. For example if there are 3 people named Jose in a room you say “Dale la sopa a los Jose nomas.” Give the soup to the Joses only; this is because the name always refers to an individual that cannot be plural. It may sound funny but the proper way to say it is “Dame 3 Cubalibre para mis compañeros”. The rules for when to use the plural are very specific in proper Spanish. I found an example back in a previous question about “Buenas Noches.” The proper form is if you are addressing an individual you say “Que tengas una buna noche” now if you are addressing a group you say “Tengan buenas noches” it is plural because the night refers to each person so multiple people have there individual night.