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1) It's very tedious to fix; you need to create 7 duplicates of the models and assign flags to them. Let's say that this can be done quickly using a map editor, but the most tedious part begins when you need to paint the flags – for heroes like Faelan, Raileigh, and the Red Renegade, the flags need to be painted because they have unique flags. And for someone, the flag will inevitably be linked to a large piece of texture, as is the case with the Dark Elves and Necromancers.
In NHF, in addition to flags, there is also equipment, for example, Godric has a unique shield with a relief unicorn – it's a cool thing, but for each such additional detail, you need to find the geometry so that everything works (if you are going to create a mod based on it with all the flags).
2) There is no Tieru, just like there is no Agbet, on the maps there are ordinary heroes, and custom icons, names, and biographies are entered in their properties.
3) This is done with scripts; I don't know how.