Here is an update after many hours of checking into why this happend!
Firstly, this had nothing to do with CORS. Although a different issue entirely.
Secondly, the server is running WHM/CPANEL environment.
Managed by the Hosting company, the default - chosen install by the host is to omit the very important module called - exif.
exif is required to understand the file types when uploaded by mautic.
Once this was installed and activated - the remote form embed worked without any issues.
There is an open issue on github to address the minimum requirements for Mautic, as a pre-install check. I hope this is also included soon, so these type of issues are addressed at installation, rather than when the mautic app is in full production mode.
reference: Mautic allows you to install with missing PHP modules · Issue #10805 · mautic/mautic · GitHub