As far as I'm aware there's no way to modify the email.
And there's no way to have a specific email as the guest sponsor. You can only set a domain, so all users in that domain becomes sponsors.
"Please note that any user with an email from the specified domain will be able to grant access to the guests if the guests select that email. If an organization needs only specific emails, like the emails of the IT administrators, to grant access to guest users, the IT administrator team has to have emails on a different domain than the other employees of the organization and must specify that domain as the "sponsor email domain", a domain with only those few email addresses on. "
https://documentation.meraki.com/MR/Encryption_and_Authentication/Sponsored_Guest#Configuration