Unable to sale my flat due to heavy encroachments by the builder and his nephew

I purchased 1 BHK flat on top floor of a four storied building in Ulwe, Navi Mumbai some five years ago. All the papers such as completion certificate and OC (dated Oct 2013) were given to me and I started residing there in the same month of April 2014 after builder provided me possession letter. Then I was the only resident (for nearly one year) in that entire building of 17 flats. That time the top floor comprised of four flats – two 1BHK “terrace” flats (owned by builder’s relatives), one 1RK “terrace” flat (owned by builder’s employee) and one 1BHK regular flat (which I own). But soon, from April 2014 the other flat owners (builder’s relatives and employee) started converting their open terraces into rooms (RCC construction with lot of restructuring) which damaged entire one side wall and roof of my flats’ bead-room and hall. I took objection but they said they will repair it and it won’t be a problem as they are close relatives of builder and they can manage everything. They repaired my wall and roof but their work continued for nearly one and a half year during which me and my family went through hell of a time which affected my mental concentration heavily and affected my work. During that same period I complained to the CIDCO about the ongoing activity but to no result. (Our building’s parking area and the ground floor flat too is heavily restructured). Two years back (2016), I left my job returned to my hometown. I simultaneously started to find buyers for my flat. I managed to close the deal three times with prospective buyers but they somehow got the knowledge of encroachments (they could see leaking walls and faded paint) and cancelled the purchase. Then, one of my close friend said to me that I cannot sell such heavily encroached flat as it could put me in a legal trouble for keeping the prospective buyers in dark by not telling them the entire story. During this period my troubles heightened and last year I defaulted on my home loan. Now, bank is taking the possession (for which I am giving my duly consent) and auction my flat. I have told the bank the entire above story, but I have not received any response yet from them, but in their notice they have informed me that I cannot sale my property as it is now going in their custody. Now I have few questions: 1. What action can I take on the builder or his relatives? (I know it’s very late now as I did not wanted any legal trouble) 2. Can bank auction my heavily encroached and “bad” property? 3. Can there be any demolitions? If there is any, it will damage my flat majorly. 4. What legal troubles I could face in the future for handing over “bad” property to the bank? 5. What could be our societies’ role in all of this? Our society was handed over to us this month (Jul 2018) and the chairman of the society is builder’s nephew residing in my adjacent flat.