HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Question: Is there a way to determine from a save file whether it was created in HD or HD+? The save file is not password-protected.

Currently, it works like this: open the unpacked archive in a hex editor;
at the end of the file, there will be the entry HD3 (this can even be seen in Notepad) in hex bytes, it will be 48 44 33 00, immediately after this entry is the version number, for example, C0 8A 37 00 - this is 3640000, i.e., 3.64, and after that, there are a few more bytes until the end of the file (their number depends on whether the chess timer is enabled).

Ideally, the save file format should be changed to HD+ to ensure incompatibility with the original. But I had an idea to avoid incompatibility: theoretically, it could be done so that when playing without HD+,
the player would receive a message every day that the game should be played on HD+.

Request: Make a quick battle in single-player mode that can be replayed infinitely without HD+. HD+ is prohibited in offline tournaments, and this feature is more suitable for them.

Unfortunately, I will not do this.
To create an infinitely replayable battle in HD+, I only needed to add one line of code because there was already a replayable quick battle. Moreover, the replayable quick battle in HD+ is without bugs, without magic, and with modified randomness. If I were to do it for the non-plus version, I would have to remove all of this from the code and also add a disable function when HD+ is enabled. It would take 100500 times more work.
And why shouldn't HD+ be more attractive for holding offline tournaments on it?

And yes, I still don't understand the situation with the autosave. Why is such a useful feature hidden in a text file? It would be logical to move it to the interface.

Because an ordinary player who is not a competitive player does not need daily saves at all; they need a save at the beginning of the day or 7 saves for the days of the week (which is currently not available in HD).
Maybe I'll make a plugin that adds an option from the text file to the game..., although I'm not sure if I'll have time.