The issue is that the Google Cloud console does not seem to be able to identify your account's email. You should try changing the primary email associated with the project:
change the primary email associated with the project
In order to change the email associated with your project, you must first add another account with the email you want set as the primary email to your owners. This is set in Permissions > Add Member. After you have added another user to the permissions, they must then access the project in order to set their email as the project's email address.
Hopefully changing the email address will fix your issue.
As you mentioned, you can only set the email address to one associated with the currently signed in user. In your case, when you added your iCloud email address and responded to the email, Google detected who you were signed in as identified that it was the same account that was already associated with the project.
You must add an additional (Google) account in order to add a collaborator and change the associated email address.
A final thing to check is that an email address is set in the top of the consent screen page. This is the first item you can set in the form and is a drop down.
Finally, if all else fails, you can try configuring your project from the classic Google APIs console.