I make ModPacks, but only for myself. They dont take too long, if your not doing something big like the Roman mod. I use modpacks, but not really in scenario design. But it would make that scenario a whole lot better if you did (only if mod was good though).
1. Modpacks take from a day (minor changes) to a couple of monthes (like Roman mod, adding a whole new civ from Dark Age to Imp.)
2. Nope, now that MPS was created its easy as double clicking a file. Before it was quite harder.
3. Not worth it if your just changing the colors or something. Unless its just for your self, because no one else would download it unless it VERY good. But the Carnage mod that Ronin did was very little, but alot of people d/led it becuase it was good.
4. Modpacks will make you have to reinstall AoK:TC if you dont backup the .DRS (only if you want to take the mod out).