Is there a way to resolve input from Forms to the Active Directory ie. check if a user has provided a valid email adress in a text field?
Assuming you are not in a government tenant, you can pretty easily trigger a Flow in Power Automate when a Form is submitted (this trigger is not available yet to gov subscribers), but there is currently no way to execute it as a validation step before the Form is submitted.
Once your Flow is triggered, there is an O365 "Search for Users" action that can take just an email address as search input, see screenshot:
Unfortunately, even if that search finds no user with that email address, there really isn't much you can do about it. You could potentially notify an administrator that an invalid email response was received, but if it was invalid, then you can't notify the submitter that they made a mistake, and Power Automate does not currently have any actions to modify or invalidate the Form submission.