I'm a bit confused with this sentence, it seems sometimes I can leave out the article but here I need to include the article 'O' for it to be accepted. Is 'Qual é meu quarto' incorrect?
I'm not sure if it is grammaticaly incorrect to say "qual é meu quarto" but it doesn't sound right. You are asking for a specific room and the "artigo definido" is desireable. I know the "meu" is already kind of specific but Portuguese is sometimes redundant.
Omitting the pronoun would make it sound unnatural. It would be like saying in English "which room is my?".