For the flats sold before completion certificate tax needs to be paid now as they wont fall in 45(5A) and that would be considered as sale of your rights in the property. It can be considered as sale of land but everything depends on your agreement.
And since the sale has been done before April 2020 you can take cost for 2001 as FMV but any sale after April 2020, cost would be restricted to Circle rate or stamp duty value.
Such capital gain would be long term capital gain.
Capital gain on sale of development rights would be long term capital gain and then when you sale flat it will again be long term or short term depending on the term for which you hold the house.
Since you have already sold rights your capital gain for such rights would be: sale consideration less Cost: 100 sq yd *200 *289/100.
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