Chronology of events in brief:-

2005 - Power of Attorney signed between 4 landowners and builder for construction of apartment
2006 - Customers enter into agreement with builder on the basis of approvals and PoA for purchase of individual units
2007 - 2 land owners (husband and wife) who had executed the PoA cancel the said PoA

2008 - Builder still goes ahead and executes sale deed in favor of customers for sell of individual units
2008 onwards - various court cases between builder and land owner who had cancelled the PoA wrt to agreement on original terms and conditions
2012 - home owners continuing to reside in the apartments bought by them by virtue of the sale deeds mentioned above. 
Questions: 
What is the legal status of the ownership of these apartments?

What is the procedure/process to get the status legalized incase it is illegal now?

all the sale deed after cancellation of power of attorney has no legal value. moreover, recently supreme in a judgment of surya, deprecated practice of builders selling flats on the basis of POA.  Therefore, for getting better title in the flats builder require to obtain title over land. adv khan

Usually flats are constructed not on the basis of POA,but based on development agreements. There are safety clauses in the said development agreement. Even the POA is cancelled, the agreement will save the purchasers. No need to worry. There is absolutely no problem for the purchasers.