Okay, I will post it here for anyone who comes to the same situation with the user deletion API.
block_ip: False does reliably prevents the IP from being added to screened IPs.
block_email: False should prevent the email from being added to screened_emails, However some edge cases (like spam-flagged users or TL0 new users) may still trigger email screening even if block_email is False.
This will need an extra request to unblock the email from Screened Emails.