You're off topic here, Paap, but I think I can give answers for at least some of your concerns.
Even with instant booking enabled, guests cannot reserve days that are already reserved on a listing, so you can't get double booked that way.
To contact support by email, go to https://www.airbnb.com/help/contact_us , scroll down to the bottom of the page, and click on the link 'my question is about something else'. Then there are more options to get you where you can open a support ticket.
And then here are your numbers for urgent phone support: +1-415-800-5959 +1-855-424-7262 (toll-free) -- Those are buried on the site, so you need to dig for them. The latter number is US toll free, so you can call it free from anywhere if you use Skype.
To refund reserved guests, you can use the cancel/change link/button that appears in various pages related to the reservations.
I hope it helps.