I am quite aware that cadence is an actual word and Cadance is not. But as you said, ninety percent of Ponies are named after actual things. Celestia is close to an actual word, but isn't. Luna isn't a typical English word. (It's Latin.) And the same goes for Cadance: Close to an actual word, but isn't one.
"Check and mate"? I didn't know you saw this conversation as a competition.
Yes, cadenza translates to cadence. This means that Cadance is her nickname or calling name. There is no rule stating that someone (or somepony's in this case) calling name should be a translation of their official name to the letter.
But why would you make up a word when there is a preexisting word that both makes sense in context, and is spelled the way it sounds? That's like replacing Cs with Ks, or Ss with Zs. I'm sorry, it just doesn't make any sense to me.
"Make up a word"? We're talking about names here, not actual dictionary words. Just because there's an actual word "cadence", doesn't mean that Cadance's name should be spelled that way.
It may not make sense to you, but that doesn't mean it's impossible her name is spelled Cadance. And as I said, following official merchandise, it shows that the official spelling is Cadance. Yes, there are a few pieces of merchandise that says Cadence, but similarly there are a few pieces of merchandise that says Apple Jack (instead of Applejack). You just have to look at which spelling is more prominent, and the more prominent spelling is the official real spelling. Namely: Cadance.