"Μου" can indeed stand alone and means "my". However, "δικό μου" must be used together as an expression and means "mine" "my own" it is more emphatic and it must agree with the noun in gender, case, and number.
See this comment for other info: https://forum.duolingo.com/comment/24409765
"ποιο, ποια, ποιος" are one-syllable words so don't take an accent. Since they are always used as question words they never take an accent. From what I see none of the sentences we have with "ποιο, ποιος, ποια" has an accent. ΙI've checked them but if you find any please point them out so we can edit them.