Current questions and answers regarding the map editor.

How can I control the distribution of skills and abilities?

It seems there's no direct way to do that. There's a workaround to completely prevent something undesirable – monitor when a hero gains a specific skill, use a cheat to remove it, and then the player will be automatically prompted to choose a skill again. It looks terrible and works poorly, so it's best not to bother with it.

And how can I highlight them so the player knows this is where the quest is given?

In my opinion, it's more interesting when a quest appears unexpectedly, but if you want, there are a few ways:
You can remove the "skirt" from the unit so that it stands out from ordinary monsters:

SetDisabledObjectMode(npc, DISABLED_INTERACT);
sleep(1)
SetMonsterSelectionType(npc, 0);

You can add some kind of effect above it, for example, a light from artifacts, like in the "Mercenaries" map:

PlayVisualEffect("/Effects/_(Effect)/Artefacts/General/Blue.xdb#xpointer(/Effect)", ...)

You can simply change the names of the units so that you can learn about the quest by right-clicking.