A.
Yes.
The use terms specify that the right to run Windows Server 2008
Standard in place of Windows Server 2008 Enterprise is allowed within
any operating system environment.
so that doesnt really make it any clearer I suppose...
Having just just rung UK MS licensing they say that you could run Standard ontop of a server licensed for Enterprise, or your could excersise your downgrade rights and say run server 2003 R2 ontop of enterprise 2008 (see here for downgrade info). There is an exception to this for webserver edition because it uses different licensing terms to standard or enterprise with regard CALs.
So to summarise, you cant run webserver as a guest ontop of an enterprise host!