Dear Sir,
Please find below answer for your quarries :
1. You should atleast hold this property for the next 3 years to ensure that the property fall into bucket of long term capital gain to reduce tax liability.
2. Yes you can surely sell the house property before possession and invest the said amount in another property within 2 years of sale of Bhoomi Ratna property to continue with capital gain benefit.
3. Yes builders do allow you to sell apartment before possession as they have transferred right of flats in your favour and now its upto you to take the call. It is depend upon your negotiation with the builders.
4. For capital gain purpose date of allotment will be of more important than date of posession.