Filling of returns...

Yes, September and October both months return required to be filed...

GST officer will verify your application and cancel the registration, if found proper..

After Cancellation of Registration, you are also required to file one final Return i.e GSTR 10 within 3 month's of order of cancellation. if you have any closing stock on date of cancellation than required to pay tax on same.

Time is not fixed how long it take for processing of application. You have to check regularly.