HubSpot forms and the Forms API do not support this functionality out of the box.
To do this, you would need to query HubSpot contacts with either this or this endpoint. Since these endpoints do not support CORS AJAX requests, however, you would first have to ping your external server and add authentication for these calls to HubSpot.
Considering the complexity of this setup, I recommend creating a custom form and using the Forms API rather than a native or embedded HubSpot form.
No, it is not possible to turn off automatic SSL forwarding ("Require HTTPS") for individual pages. It is possible to turn it off for the entire subdomain, but this would still not prevent some visitors from loading the page over HTTPS.
The most sustainable solution will be to configure your endpoint for HTTPS requests.
HubSpot forms and the Forms API do not support this functionality out of the box.
To do this, you would need to query HubSpot contacts with either this or this endpoint. Since these endpoints do not support CORS AJAX requests, however, you would first have to ping your external server and add authentication for these calls to HubSpot.
Considering the complexity of this setup, I recommend creating a custom form and using the Forms API rather than a native or embedded HubSpot form.