Specific to your case, I think a workflow following form submission along these lines:
Starts with an approval step and uses a one click approval link in the assignee notification that gets mapped to the email field. This would validate their email address.
A 2nd approval step mapped to the administrator role (or specific users) with an approval policy of any (1 admin approving being enough to continue the workflow). See configuring an approval policy for more details.
The step to create the user. The registration feed is the same as you’re used to now, just a step in the workflow to define when it should be executed.