A number of questions regarding map creation.

It's quite possible, it's just that they rarely make such things, but sometimes you can still find them.

I could have tried it on a smaller scale, but I decided that if I'm going to do it, it should be something epic. Of course, it's probably not ideal for a full-fledged multiplayer game due to its size, but there will probably be some fans. As it is, the idea is to have several storylines that can be played through in single-player mode. That is, if there is a choice of who to play as, then on one map it would be possible to create several stories in the context of a general one.

TalkBoxForPlayers
This function exists in patch 3.1, and it has many functions, not just displaying a message to a specific player. If the message is small, then I think ShowFlyingSign(messageName, objectName, targetPlayerID = -1, time = 1.0); is the ideal option, because it does not require the player to take any action and is very convenient from a gameplay point of view.

It's an interesting thing, but according to the description, it requires many files. Is it necessary to specify all the links, or can you simply put empty quotes in the item with the icon? And did I understand correctly that you always need to create a file with answers, or can you somehow make it so that there is just text and no options, just "OK"? I apologize if the question is stupid, but this is my first experience working with scripts.

I don't think it's a very good idea. For multiplayer, it's better for players to be more or less equal. In Heroes 3, which you said you played, there is "The New World," a good example of a multiplayer map where different players have different strategies and quests (although the quests there are limited to the prophet's huts, but still). And three against one will still be unbalanced, even if the fourth player is clearly stronger than the other three.

Hmm, maybe you're right. On the other hand, I already have a plot in my head. In short, once upon a time, they defeated the Necromancer, but he didn't die, but fell into a state of suspended animation, and now he has awakened. Slight amnesia does not prevent him from gaining power, which can even be realistically shown in the game, for example, if, as soon as the game starts, the lighting on the map is daytime, and after a while it changes to a Necropolis-like one. You can balance the players with strong monsters that will slow down one side and speed up the other. But, perhaps, there will also be a game with 1 against 1 or 2, or even 2 against 2. I am making the map primarily for myself and my wife.

Regarding the whirlpool. Can it be used as a teleport, entering on the surface and exiting underground?

Added after 4 hours 32 minutes
TalkBoxForPlayers

There is a problem with the function. There is no main text to the right of the icon. As I understand it, I need to add a configuration file in the Russian patch so that the function works correctly. But which pack should I put it in?

a2p1-data.pak
a2p1-texts.pak
data.pak
sound.pak
sound.pak
texts.pak