Slitherine

Can the scenarios used in the campaigns be brought over to multiplay?

Not really - they are designed as 1 player scenarios.

is it because they wont work or they would be out of balance?

If you removed the script it would work but it'd be imbalanced. Also, some AI units are placed outside of map boundaries, and are triggered as reinforcements by the script. So without the script, they would just stay there and can't be moved or killed.
I'm not sure what would happen if you did keep the script, scripting hasn't been tested much in MP yet and i'm sure a SP script would cause all kinds of weirdness.

But it is possible to use the SP maps, get rid of the script, redo all the units so they are balanced and all in map borders and then use it as a MP map. A bit more work than simply copy-paste, but shouldn't be too hard.

But it is possible to use the SP maps, get rid of the script, redo all the units so they are balanced and all in map borders and then use it as a MP map. A bit more work than simply copy-paste, but shouldn't be too hard.

I like that! It's still fun to play but nothing beats exploring a new map.
I try to do maps but I'm not very satisfied with what I managed to put together so far.