What happens to the form in the email in the first scenario (when it works as expected) when the user is offline or does not have access to the SharePoint site?
IPFS (InfoPath Form Services (the browser form)) does require access to the SharePoint server to run, I think with your case the XML is expected to be opened up in InfoPath client, or if the users opens that XML file attachment does the form open in an IE browser?
keep in mind that with the file attachment they won't be able to open it in the browser unless they are online and have access to the SharePoint site.