1. Consumer forum is most effective forum to get the necessary relief.
2.Yes you can. Even individually also the willing flat owners can file such case.
3. Yes you can. It is the fault of the developer.
4.The consumer forum will pass necessary decision on that regard.
5. He can stop maintenance service. Seek INJUNCTION FROM CONSUMER FORUM IN HTAT REGARD.
6. Consult a lawyer and do the needful on the above lines.
Feel free to contact for further help.