Questions about the Heroes 4 map editor.

Not on every map. There are random numbers, so you can set the probability of losses to, say, 3% or so. And it should be annoying.

The terrain becomes challenging.
If randomness is planned, the player has a way out in the form of save/load. :)
You can place PE (presumably, Permanent Enemies) not so densely, or near resource sources, or cities - depending on the map.
"Hostility" can be implemented by, for example, increasing the number of neutrals.
Or make the passages paid. Or... there are many options.