2. The original parent deed can be held by one of the joint buyers at the time of partition of the jointly purchased property.
A mention about the retention of the original parent deed with either of them and the other having a notarised certified copy of the registered document, in the registered partition deed would solve this problem.
Since the partition deed to be drawn subsequently, would be be by a registered deed, the contents or the recitals as mentioned above would be recognised as legally valid by which the new prospective buyer need not worry about not having the original document.
3. You cannot mention about each other's portion in the registered sale deed executed in favor of both of you by the vendor.
Since you both intend to partition the property subsequent to this purchase, demarcation of boundaries mentioned in the registered partition deed would be sufficient.
4. You do not worry about non-existing rules or restrictions now itself.
First you purchase the property jointly.
After that you can decide to partition the property by a registered partition deed immediately whenever it would be feasible.
Association that may be formed at a later stage cannot object or restrict the already partitioned property, hence you may concentrate only on what is in your hand now.
6. The developer cannot object o your intention to partition or to sell the property.
If he is charging maintenance charges, then let him give NOC after collecting the charges for transfer, if any, but without an association formed, such levies by the builder is not legally maintainable.
7. If the plot now proposed to purchase is under A khata under form 9, then the partitioned property also will fall under A khata category only.
8. If the documents held in your possession are legally valid and also if your plan to construct the structure is approved then there is no question of any issue in this regard at a later stage.