You have to complain to the BMC for demolishing these unauthorized constructions 

That will obviously create a rift between you and the persons who would be affected by your complaint 

I also suggest that the society engage a good architect and take a feasibility report for the proposed redevelopment and ascertain whether what the builder claims is indeed true or not 

Having a feasibility report will bring in clarity and if there is indeed some redevelopment potential then the society will have an upper hand in the negotiations with the builder 

If the report says that the extra constructions have consumed FSI more than the permissible FSI then no point going for regularizing of those additions since that will not change the FSI consumption status of the building. 

The building needs to have consumed FSI which would be economically viable for any builder to provide to the society members in the rehab building alongwith other redevelopment benefits like incentive FSI and fungible FSI if the same are applicable depending on the regulation of the DCPR under which the building would be redeveloped 

But first and foremost have the feasibility checked and ascertain what incentive a builder can get if he takes up your building for redevelopment 

You need to seek eviction of the same through court Or MHADA as per the competent authority of the property

Issue legal notice to commercial entities to vacate premises reserved for parking slots 

if they refuse file eviction suit against them 

also issue notice to those who have encroached on terraces 

1) occupation is illegal as contrary to sanctioned plans 

2) complain to BMC against commercial entities carrying on commercial activity in parking area 

3) if society does not take action individual member  can complain to BMC , issue legal notice , take legal proceedings 

Even if maintenance is taken it can't be legal if it's primarily illegal

It appears that there are lot of deviation in the construction violating the approved plans.

You have mentioned that the builder stated that the 6 illegal entities have increased the FSI consumed  hence making the redevelopment impossible.

Have you confirmed this message from an independent architect ? i.e., have you destined a feasibility report for the proposed redevelopment and ascertain whether what the builder claims is indeed true or not?

If the feasibility report says otherwise then you do not have to go with the same builder and may engage another builder who can be accommodated with the facts for this redevelopment. 

Alternately the society can issue a legal notice to the commercial entities to vacate the premises now being used for commercial activities which is illegal.

If they refuse or come out with a different story from their side, you may approach court of law with a suit for eviction against them for illegally occupying the space for commercial activities which is otherwise meant for parking space. 

You can send a legal notice to the illegally constructed flat owners who have occupied them in the terrace which is an illegal construction.

A complaint also can be given to the competent authorities about such illegal structures and recommend for their demolition for various obvious reasons, but this action can be taken by the legally authorised body i.e., the society and not as an individual. 

1. The occupants may certainly take defence on that basis, hence the society should in an emergency meeting should declare that the car parking space occupied by certain entities are illegal and they are to be evicted immediately besides stop taking the maintenance charges from them and also to disconnect the electricity and water supply connections and any other amenities provided to them to run these commercial establishments  illegally.

2. The other architect who opined about the illegality as well as possible legal action from the competent authorities are facts, hence it is advisable to not venture into such illegal activities. 

3. The society as a legal entity should take a decision on this.

If the society fails to arrive at any consensus then as an individual you can make a complaint to the Municipal authorities about such illegal constructions as well illegal commercial establishment in the space meant for car parking. 

If the authorities do not take any proper legal steps to rectify the same you may file a suit for mandatory injunction against them seeking proper relief and remedy to this situation. 

1. the contention of the occupants of the commercial premises is not legally right. Just because the society collects maintenance from them does not mean that their structures become legal. If that were so then even BMC collects property tax from illegal slum dwellers...do those slums become legal then?

2. i do not agree with the view of the PMC that the society will get incentive FSI whether the premises are legal or not. Reg. 33 7 B clearly uses the term 'existing authorised BUA'....on which the incentive FSI is calculated. So if the FSI used in unauthorised then no incentive FSI can be claimed on such illegal FSI. If BMC learns about the illegal consumption of FSI, even then the society would be in trouble, particularly the persons who are in occupation of the illegal enclosures

3. just because these persons are 50% in strength, does not mean that they can thwart any measure taken by the society for undoing the illegal and unauthorised constructions and the illegal premises occupied by such occupants. There can be no inclusion of votes of society members for the purpose of passing a final resolution by the society where the votes are by persons who are guilty of breaking the law