1. GST filing depends on the accounting. If the consideration of the building is linked to construction linked plan then you were required to raise the invoices at the time of completion of construction at each stage and filed the GST returns accordingly. If the consideration is linked to payment then the invoice could have been generated at the time of each payment.
2. It can be done only in the return but take the help of CA because it is a very crucial area.
3. No need to mention in the return.
4. of course.