All questions regarding mapmaking for Heroes of Might and Magic 3: The Shadow of Death.

Yes, to prevent a week of real-world events or a month of plague, the player needs to play with the HD mod and the SOD_SP plugin enabled, and also configure its settings. It would be best to inform the player about this right from the first event.

How do I make an enemy hero start moving at a specific moment?

There are several ways to do this. But instead of making them start moving, you can have them remove a guard at a designated location. The guard can require the player to bring N specific creatures (preferably neutral creatures that can only be recruited in a dwelling). Or, they can require resources, but then you need to disable the market (otherwise, the player can exchange resources and leave earlier). A more complex method: in the desired location, there is a town without a tavern and a hero, and on a specific day, a tavern is built. To change the appearance of an object in the extended editor, do the following: place the object that you want to reskin, remember (copy) the model name (the "Image" field), for example, for a ground cartographer, it's AVXmaps0.def, then place the desired object in the correct location, open its properties, and paste the name of the original object. Click OK, and you will get the desired result. But it is better not to change the appearance of objects without a real need, so as not to confuse players.