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

The suggestion to describe some additional basics is very good, but I won't do it because I simply won't be able to handle it. There should already be a lot of information here without which it is impossible to do anything. You yourself should think that even a beginner needs to have at least a brief description of how to work with in-game archives, and that's practically a whole section. Otherwise, a person simply won't be able to test the finished model.

Well, I just made a suggestion, nothing more. It seemed to me that working with game archives should be something a person who has taken on working with models already knows, and the second is impossible without the first. On the other hand, you might be right, it probably wouldn't be superfluous.

By the way, it seems that Esto100nec1 had a guide on in-game archives, maybe to save time and effort, it would be worth just adding a link to your guide? Or even quote the main points from there, if Esto100nec1 doesn't mind.

If there is a desire to describe the aforementioned Max functions in a simple and clear way, I will be very happy to add this to the tutorial in the "additional information" section. And as a separate topic, it would also look quite good. Something like "useful Max functionality for beginners". I myself would be happy to read about skinning, because I haven't really done it properly in Max yet. Otherwise, beginners will simply have to Google and look for tutorials. In the end, this information is not rare, unlike information about importing a model into Heroes 5.

Okay, as soon as I have the opportunity, I'll send you a document in which I will describe those simple editing options that I was talking about, and I'll show it in screenshots. I think you'll figure out how to make it more accessible for the viewer.
As for skinning, this is clearly not something simple for a beginner. I don't think it's worth bothering with it, because it's unlikely that anyone will deal with it and work with it. And it is not necessary to attach an object to a model in Max through all the things related to skinning, there is a simpler way in just a few clicks. I will describe it as well.

When the main part is ready, I will definitely write about fixing the movement animation in "additional information".

Can you tell me, is this adjusting the model's animation by coordinates, time, cycle length, and playback speed, or is it a specific fix that allows you to use the movement animation in place, but only in the context of Heroes 5?

I will definitely write in the additional info how to edit the move animation and how to quickly split the model into several parts if it has several textures.

This is probably a "side effect" of converting to SMD, when after conversion all the meshes/materials are merged into 1?

"mirror unfolding" what kind of beast is that, may I ask?

I meant a two-sided material, I just didn't express it correctly. I'm not very good at explaining technical details.
In general, when a texture should be applied to 2 sides of one plane, and not to 1, as by default.

I currently have a problem where the inside of the guard's helmet is transparent from the face side.

Here, it seems we are talking about the same thing...

And in Heroes, is there a function to make a two-sided material? Does anyone know how such functions work from the point of view of the engine, does it multiply the number of polygons calculated in the game or not? (you can find out by cutting it directly during the game with some ripper and looking at it in Max.) If the polygons are not multiplied, then it is more appropriate to use this function, if they are multiplied, then it is more appropriate to select only the necessary polygons and manually copy and unfold them in Max.

I don't think it's implemented, because I've tried all the possible material parameters, but I couldn't achieve the desired result. But, as I already said, this can be fixed with some "tricks" using the "Shell" modifier in 3ds Max, at least on the models I've worked with. It is also likely that this can be fixed by duplicating and mirroring the polygons.