So I've found other recommendations of this, including one that explained that the 14-digit number produced by leaving off the last digit of the IMEI is actually an MEID.
My fear is that the email you receive after entering only the first 14 digits is just an acknowledgement that you submitted the form and not a confirmation that everything is in order. So with my luck, 15 days from now, after the last day to submit the form, they would come back and tell me my submission was invalid.