I can't fully confirm this one way or another, but I don't think that it will send the email if you delete your account before the time it is scheduled to send it out. It's a bit like putting a package in a mailbox so that it can be picked up by the mailman for delivery next morning, but blowing up the mailbox with the package still inside it before morning.
It is true that once an email is sent to an address you cannot stop it from being delivered, but scheduled delivery controls when it is sent. It isn't delivering the email to another mailbox and only revealing it later at the scheduled time, it starts the delivery process at the scheduled time. So unless Google has built a feature into Gmail specifically to gather scheduled messages from deleted accounts so that it can send them at a later date I don't think it's going to work.