Why isn't there support for all versions of the game yet, including "Chronicles"?

Well, I was hoping to see comments from the developer, but... it seems he already considers himself "close to the gods" and doesn't even appear on the sites he himself indicated...

Indeed, where do these judgments come from? Forums, in principle, have outlived their usefulness; they have been overtaken by messengers and social networks. Unfortunately, I personally prefer forum communication.
Why should I monitor topics in which only 1 new post appears per year, and in most cases, there are already answers from other people?
The HD mod is my hobby, not my job. I need to feed my family, not sit around and wait for someone to ask me a question on the forum.
Moreover, the HD mod itself is already a huge monster of 340,000 lines of code. Such projects are usually professionally managed by a team of developers on a salary. And I am alone. Its support and development already leave no time for conversations. My website about the HD mod hasn't been updated for several years - because there is no time.
And yes, for most of the mod's development history, I have focused primarily on online players because online players are the most consistent, loyal, and responsive audience. A single-player might play once a year and be satisfied, while online players play constantly.

That's what I don't understand - why does this mod work for some individual add-ons, but not for... most of them? How was this chosen? "By trial and error"?

It was chosen like this. For SoD, a large base of research and modification had already been developed by the creators of WoG and their followers; it would be foolish to abandon all of this legacy.
Why only SoD? Because SoD version 3.2 is the pinnacle of the development of the official Heroes 3. In it, there is everything that exists in other versions + more. The complete version 4.0 does not differ in terms of functionality and the presence of bug fixes from SoD 3.2.
Although Chronicles was released after SoD, it is a cut-down version. It does not have single-player scenarios!, it does not have a map generator!!!, it does not have multiplayer!!!. Why are they needed? Modifying Chronicles or Armageddon's Blade would require redoing a lot of work on researching and modifying the code. Why? If you can play any mission or campaign in the series in SoD 3.2?

So, the problem, to put it mildly, is not solved at all and is still more than relevant...

Believe me, making an HD mod for Chronicles is a thousand times harder and takes longer than starting to play them and reaching the save points again.
It may seem to me, but I feel indignation and even resentment in your messages. Go ahead and make an HD mod for Chronicles yourself. It's not that hard!