GST For E-commerce:

An E-commerce company having branches in Delhi, Punjab, Bangalore and Pune. This company has registration in Delhi and Punjab only as an E-commerce operator for deducting TCS and Normal registration as well. There is no set up in Bangalore and Pune. An E-commerce is mandatorily required to take registration in every state for deducting TCS and can show head office as its place of business. By this logic, we should take registration in Bangalore and Pune as an e-commerce for deducting TCS by showing Delhi(Head office) as Place of business. Now, I have some questions:

1. Is it mandatory to take Normal GST registration as well in Bangalore and Pune i.e for filing GSTR-1 and GSTR-3B?
2. If we take registration, then we have no set-up in Bangalore and Pune. How to take the registration?
3. If not mandatory, from where should we file GST returns i.e GSTR-1 and GSTR-3B, from Delhi itself?

Please give me reference of any section. It would be helpful.