Until there are sufficient members required by law, a society or an association of flat owners cannot be formed, hence the builder is responsible for maintenance until then. The minimum members are to wait for other members to get in
If the khatha is awaited for the akrama and sakrama to become law, then you may have to wait too long because the government announced the scheme long back but have not taken any step to enforce it though there was an incidence earlier wherein it was not successful. The assurance given by the builder in a non judicial stamp paper even duly notarised will not have any legal value when it is required to be enforced by law. You should have taken legal opinion before purchasing the flat constructed on revenue sites without proper khatha or with 100% or full deviation, because the builder will thousands before purchasing just to fleece the buyers, but it becomes the duty of the buyer to verify and confirm before entering into the venture.
Taking the possession certificate alone will not solve the problems including the issues of amenities like lift, generator or electricity etc. However, better get the possession and the property duly registered and then you may go for legal battles on all the incomplete issue by drawing the builder to consumer forum for all the deficiencies and if possible to the civil court to for recovery aspects also.