I look forward to the day I meet Mary in heaven. I've always wondered if she knew who Jesus really was before the resurrection. I'm inclined to believe she thought He was a prophet, maybe the Messiah, but I doubt she had any idea He was God in man.
My opinion is, you're correct, but I am not 100% sure. There is no doubt Jesus' half brothers did not believe in Him, according to John 7, but I think they became believers later. Better late than never.