Tenants right

Local laws apply.

It is not possible for landlord to demolish the chawl and you are not informed while you are stated to be residing in the chawl.

It would be appropriate to consult and engage a local prudent lawyer for appreciation of facts / documents, professional advise and necessary proceeding.

Yes better to consult local lawyer and act accordingly

In Mumbai there is a Rent Act in place according to the Act a tenanted premises can not be demolished without the legal tenant's consent and without providing alternate accommodation. 

If The premises are reasonably and bona fide required for occupation by the landlord himself. This is also true for carrying out of repairs, which are not possible without the premises being vacated and demolished. If a tenant fails to comply with the order of eviction within 30 days of the date on which it has become final, a competent authority may evict them using such force as may be necessary.

Yes, the action is not lawful. Remedy lies in approaching the jurisdictional court.

The landlord cannot demolish the rented premise without giving notice or the consent of the tenant or a court order in this regard.

The act is illegal and unlawful you may approach court with a suit for permanent injunction  and other remedies.