Aside from not being able to create a new mission when entirely off line
You should be able create missions when totally offline, but you may not have the right (or any) map area for the location your mission is in.
There seems to be no real ability to zoom more than 1 level and to pan to Milwaukee would take an hour at that zoom level.
Currently you can only cache maps for rather small areas and limited zoom levels. Maps take up a lot of space on your disk and the MM server too! The size is limited so not to overwhelm the MM server space as the maps have to be save on our server before they are sent to your cache. The MM Maps screen tries to limit your "world" to the areas of the maps that have been cached. Maybe it should let you go out side that space even if there are no maps?
Second - is there any possibility of adding the ability to load GPX files while off line?
Yes that's possible. I'll add that to the feature request list.
Regards,
Mike