1. It can be single account or a combination of both. Do note that the total should not go below even for 1 day in the last 90 days before the application. Also, cross verify the exchange rate to convert the INR to GBP using oanda.com (official site used by home office). There's nothing wrong in having a bit more in the account.
2. Not a mandatory document. But, I've heard some applicants submitting it. Do some reading around, you may find those cases.
3. For the complete set of documents, please read the guidance document thoroughly. You will also find many successful dependent applicant cases in this forum which you can go through.
https://www.gov.uk/tier-2-general/family-members