In reverse order:
Does your 265w have the original OEM maps on internal memory? If so, I'm not sure why a 3d map is present unless it was added later:
It can't see 3d terrain or buildings/landmarks. Remove that map.
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If the gmapprom.img is OEM or official you could back it up (along with any associated gmapprom.unl file if present) and then remove from the unit. My preference is to leave oem/official maps on the unit, but deselect them, and have current 'unofficial' maps on sd, not on the unit. If the gmapprom is 'unofficial' just delete. Don't remove gmapbmap (basemap) or tmaptz (time zone) image files. The advantage of putting unofficial maps on sd is that if something goes wrong with them or they cause problems with the functioning of the unit, you just take the card out. Don't have maps of different versions covering the same areas selected together or you'll have possible problems. Overlapping maps of the same version are fine select together.
Not quite sure what you mean 'are the southern States covered in the [lower]49 States map'? Unless the Civil War is still active in the Deep South (OK, of course it is), the lower 49 included them. And the Southern Sates & Mexico means what it says: have a look at the coverage maps in pietu post #2:
EDIT: I forgot this question: 'is mapsource the only way to combine or edit the map?', but it's actually sorta covered in my earlier reply. You can also use BaseCamp/MapInstall to select parts of a map from a gmap folder to send to unit or sd, and also to combine maps to send.