From what I understand the "est-ce que" a. makes the sentence more formal, and b. indicates right away to the listener that a question is being asked.
But yes, I'm sure either "es-tu nouveau?" or tu-es nouveau?" said with a rising inflection would be fine for informal/casual use.
because you cant translate everything word by word. it doesnt work like that. you have to understand why the sentence is made like that. in this case, when you hear "est-ce que" you know its a question, and the thing that comes after is the main stuff. and also in english it doesnt make that much sense either, the sentence "is it that you are new?" so yeah, thats why it didnt accept it.
Actually, depending upon the context, it could make sense in English, albeit not commonly. I do get that it primarily signals a question in French, and that it is not intended to be translated literally. I submitted it as a matter of checking to see if it would accept it, not that I would have translated it in this way. Still I kinda sorta expected that it would accept the literal translation.
in such cases, I think it is better to treat such cases as new vocabulary rather than breaking it down... "est-ce que" means "are" in a question form! so in cases like "est-ce que tu es" you disregard "es" and you get "are you"
another case that can be treated as the same is "Qu'est-ce que" means "what are?"
as you go forward, you may find "est-ce que" and "Qu'est-ce que" is being used in a totally different way, and you end up to have more than one meaning.... which is common in many languages and they way to know the exact meaning would be the context....
take a simpler example: "lit" can mean "bed" and can mean " reading"...
you can learn to accept by sounding "est-ce que" and "Qu'est-ce que" in google ,or whatever, they sound much easier than the way its written
They have different meanings in english, in that sentence, "that" can be the same as because, rather than just asking "are you new". Also, 'tu es' vs 'vous etes' and 'ils/elles/ons sont' the adjective nouveau would have to be modified accordingly (masculine/feminine, plurality). Using the phrase 'is it that' can be perfectly fine to help you understand the construction, but the meanings are slightly different.