1) It depends on your scenery load order. If USOrthophotos is below Aspen Mesh, you will see the Aspen mesh, but not the USOrthophotos mesh.
2) Again, you have to order your scenery.ini in a way that the mesh you want to use for a given region is above any other mesh, but below the airport.
3) Airport comes first, Autogen second, Mesh last.
4) No, HD Mesh is a freeware mesh by Andras Fabian. It does not use photos, but is a higher resolution version of the default mesh. You can only use one mesh -- either the default mesh, or the HD mesh, or any orthophoto m