The OpenIn parameter does apply to existing XML as well as XSNs. So does Source. Open an existing XML in the browser (from a library configured to open in the browser) and you'll see the source param in the URL. The MSDN page I linked to explicitly states in the section on OpenIn:
If a Source query parameter is specified, the default value is Browser.
Thus, if in the URL to the XML you include in your email, you include OpenIn=Browser, the XML should open in Browser. Or include Source, if that is your preference.
But a URL like this:
http://<server>/_layouts/FormServer.aspx?XmlLocation=/<library>/<formName>.xml&OpenIn=Browser
Is going to open in the Browser. If you want to redirect on close, use Source instead - if you include Source, your XML will open in the Browser. Again, though, it won't close Internet Explorer when you close the form.