Rotating maps isn't possible. In theory you could, to some degree, rotate/flip mulitple map objects with a custom Mapper script, but Fallout graphics isn't really prepared with such scenario in mind. If you need to see what's behind a wall, you can hide/show walls using "show" buttons in Mapper.
Yes, you can copy objects and paste them into a second map, simply switch to the other map and paste. Just remember stuff is pasted at the same coordinates, so if you paste objects into a smaller map you may have to move it first or it won't fit on the map.
Dead critters I don't remember exactly how you do it, but you can open some existing map that has them and check the properties. In 2238 there was one corpse in the se_truck.fomap map (inside the warehouse). It's a matter of changing a property value from 0 to 2, or something like that.