At which point of destructor call the object ceases to exist?

At which point of destructor call the object ceases to exist?

The lifetime of the object is ended by the call to its destructor. Within the destructor body, the sub-objects are still alive and member functions may be called. After the destructor body, the sub objects are destroyed.

if it's legal to call an object's function inside its destructor or not

It is legal.

Note however that calling a virtual function works differently than one might expect.