To answer some of your questions, There are plans beyond 3999 (for 1 Gbps) with higher data caps such as 20 TB, 30 TB,etc. It was confirmed by the Sales person who initiated the connection. I can only see next two upgrades on my account, though.
The sales team usually needs any one of
business proofs such as GST, Incorporation certificate, and PAN.
I had a B2C connection (Airtel Black) on the same premise and got it disconnected to get a new one. I got it installed before the another connection was in process of disconnection. You cannot convert the segment from B2C to B2B, as confirmed by me personally on e-mail.
You get one month extra if you pay for minimum of 3 months on installation. After that, It is a postpaid connection paid monthly by you. So It is beneficial to pay 3+1 on installation. You will also get GST input and free Static IP. So overall cheaper since you won't pay 299+GST every month extra with higher data caps.
Furthermore, You won't, most likely, get a call from Airtel If you do sales enquiry on their website. I waited for 4 days before calling them on the number mentioned for Airtel for Business. I was provided with a point of sales contact details in my area by the customer care executive.
I then called this person who only sees the B2B connections, and he got me started same day. Within three hours, my connection was active and running.
You get the same
ONT device with no OTT included. I have been using Airtel Black for my family SIMs and am getting few OTT benefits through that route.
Also, Minimum commitment period is 90 days. So You won't be eligible for refund I think. I was using the B2C for few years and have been satisfied so far, so It didn't matter.