1&2&4) I'm afraid you edge doesnt handle more than one gmapsupp.img per sd
you can join more than a gmapsupp.img with Gmaptool
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The new firmwares for the 705 support at least 8 GB cards, but I think that maps are still limited to 4 GB per files (can't confirm) and probably you are also limited to a certain number of map tiles-(2025 map segments?)

So it's better to have more than one sd and swap them or use Mapsource to create an img containing the regions you need ,respecting tiles limit .
Alternatively nothing can stop you storing several maps on the same sd card, only the active one must be called gmapsupp.img. and just rename the others to another name when you're not using them.When you need to show one, change the names.

3)No it's better to keep your basemap, read
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I asked a lot of "stupid questions" before having a lil knowlegde about these devices and I'm still asking