Which GST forms to be filled by online seller

I sell chocolate products online (zomato, Amazon etc.).

when I sell on zomato - it is delivery only restaurant - GST 5% - No ITC

When I sell on Amazon (Interstate)  - it is sold as product - GST 18% - ITC allowed

I am running my unit in rented place. Owner does not have GST no. so I don't get GST taxed rent receipt. 

I have following queries. Your guidance will help me.

1) Shd I approximately take ITC, based on assumption that x% will be sold on Amazon and y% will be sold on zomato, when I purchase my Input material.(Because there will be only 1 invoice when I buy input material)

2) Amazon and Zomato charge commission, convenience fees and also apply CGST, SGST. Such charges on Amazon, I can take ITC. Zomato, I can not take - Is it correct?

3) Is there anyway I can get some credit for the rent I am paying?

4) I need to fill GSTR3B for paying tax, based on Invoices I receive from Zomato, Amazon for the month. Is it correct?

5) I sell my product online, so which GSTR form should I fill, GSTR1 or GSTR2. I should give details of only zomato and Amazon invoices or each transaction of each customer to be filled? I am confused here and seek your help.

6) My turnover is about Rs. 20 lacs, so when should I file return? (return means, GSTR1 or 2 or 3 - kindly let me know).

Looking forward to your guidance