If your email from TurboTax says it has been accepted, then we received that notification from the IRS. If it says transmitted or received, then there will be another email when it has been accepted or rejected.
If you sign on to TurboTax and go to the lower left to File an Extension, it should display that you have already applied and show you the status.
Yes, it is. Also, I don't think IRS has anything to acknowledge extension.
6 month extensions are automatic with the IRS. Note: it is an extension to file, not an extension to pay. You must estimate if you owe anything and pay that with the extension or be subject to penalty which adds up fast each month. It is based on a percentage of what you owe so if it is zero or a very small amount, don't worry about it.
All of the emails you receive regarding 'submitted' or 'accepted' - tax returns or extensions come from TurboTax. The IRS does not send acceptance emails or text messages. @trusharp
See this IRS help page.