1 - there's a readme with information on the map editor.

2 - Same as before. Drop 1 unit on the other. Only this tim when you drop a 40 men unit on a 50 men unit you will end up with one unit with 60 and one unit with 30. You can also retrain damaged units (button left hand bottom corner) so replenish their losses.

3 - A useless unit only eats upkeep (I guess). If they only cost money you can disband them.