Delay in possession and allotment of property to some other buyer

I hv booked a flat at NaviMumbai in 5/2015 (Under construction) for residential purpose. I received summons frm court stating that the flat has been alloted to another person prior to my allotment. He is an investor & has also invested in 15 flats. He has filed a case agaist our builder to deregister our flat & claim to have right over it. I hv compeleted all legal procedures, taxes & almost paid total consideration. The another person has nothing apart frm allotment letter. On discussion abt this with builder we were told that he has returned all the invested amt & has assured us that he will sort the matter positively. On enquiry of possession he is ready to give the keys inspite of pendind works(installation of lights, lift,water supply, electrical meter box; fire NOC, etc.). He has assured us in written to undertake cost of maintenance until the project receives OC. My qstn is considering the above situation should I accept the keys before case is solved and before recieving OC?