Just beware that an export will not get alerts, recycle bin, Web parts
personalization and probably more. Whereas -backup  is full fidelity (when SC is
locked), but scoped to site  collection.

To export/import a single  site, I think you must use sharepoint designer. Maybe
an SPD expert here will know.

Yes, that's correct.

SharePoint Designer includes a menu option site  > Administration >
Backup Web Site...

This is exactly the same as stsadm  -o export (uses the same APIs
under the covers), except:

* The export file(s) are downloaded through the network to the
user's desktop
* The user has the option to include or exclude sub-sites
* The user does not have the option to include security