Guide on adding a new model, animation, and creature to HoMMV

Polygons may not be displayed for the following reasons:
1) They have a "too small" UV map; this can be fixed by simply moving the vertices so that the polygon's UV map is larger.
2) The polygons have, let's say, a more understandable "linear 2D UV map," as in the example with the cavalry unit; this can be fixed by completely redoing the UV map of the non-displaying polygons.
In very rare cases, retriangulation helps: you take a polygon that is not displaying and merge it with a neighboring one into a square, and then back into triangles. It is important that the square is divided into triangles differently; if it is divided into the same triangles, there will be no effect. I used this method once for the half-furies, half-lancers from Heroes 7; a polygon was not displaying on the left glove.

Added 1 minute ago

Added 6 minutes ago
Do you have any thoughts on this? Which of the two methods do you think is better for fixing this problem? (this question is also for FrozenSoul)

The creatures from Heroes 7 do not have a back side; in Unreal Engine, you do not need to duplicate the back side.

Added 1 minute ago
But for Heroes 5, you need to duplicate it, and this does not apply to the problem described above.