1 Interest paid by you on the housing loan till the completion of construction can be capitalised and treated as part of the cost of acquisition. However, interest paid after the completion of the construction cannot be treated as cost of acquisition. Such interest could have been claimed as deduction u/s 24.
2 If the property sold is a residential house, it is sufficient if you invest only capital gains in acquiring another residential house to claim exemption u/s 54. However, if the property sold is not a residential house, you can claim exemption u/s 54F by investing the sale proceeds in acquisition of a residential house, subject to fulfillment of conditions specified therein. You can also claim exemption u/s 54 EC if you invest the sale proceeds in capital gains bonds, subject to fulfillment of conditions specified therein. If your investment u/s 54 is less than capital gains or your investment u/s 54F or 54EC is less than the sale proceeds, your exemption will be proportionate to the investment.
3 If you can sell the property in April or thereafter, definitely your indexed cost of acquisition will be more than what it is till March. In terms of time available for investment, it is with reference to the date of sale and not with reference to the financial year.