Buying a property through GPA

Hi, I am planning to buy an apartment in Bangalore from a known builder. Upon checking the legal documents provided, the builder has agreed a JDA with land owner and there is a GPA given to builder to sell the apartment, this GPA is registered as well. If I am purchasing the apartment by paying the builder as per a sale agreement will it stand valid? Will I be allowed to register the sale deed? Also, the current land owner has been given a GPA(1) by original owners of the property, and this GPA(1) was notary attested in 2002. And in 2004 an absolute sale deed to sell the property to current owner for a consideration and stamp duty been paid and that absolute sale deed was registered also. However, In this absolute sale deed the seller and buyer are same person (current land owner) as the original owners mentioned in GPA(1) is not signed the absolute sale deed and signature is made by GPA Holder (current land owner) in behalf of original owners. Is this sale deed valid considering this is already registered in sub registrar office.? The khata certificate is in the name of current land owner, bbmp tax paid receipts are also in the name of current land owner. Can the ownership of current land owner be challenged in future if the GPA(1) has minors? Also, the mutation records shows current land owner name, (original owners name changed to current owner) however the mode of possession in MR record is blank. (Usually it shows by purchase, by birth etc..)