1.Yes,do take possession first and then take legal recourse.
2.once you get possession send legalnotice for execution and registration of a deed of conveyance.
If it is not done then you can file case before the consume forum whwrein you will get not only the order for registration of sale deed but will be awarded with damages and compensation as well.
There is no demerits if you take possession of the flat and then file case for registration of sale deed.