The words can be the same word. Casting Decisions, Decisions or Monitor Monitor will cause you to scry 2.

Questionable Cuisine is not alliterative. Celestial Crusader is. It’s true that in the real world, alliteration usually refers to the repetition of sounds at the beginnings of words, but it’s also true that porcupines are usually not very aerodynamic, so let’s not get hung up on reality, okay?

An alliterative spell or creature can have other words in its name, capitalized or not, as long as it has two or more capitalized words that begin with the same letter. The two words don’t even need to be adjacent to one another.