another problem: when I enter Edit Script, the editor crashes. What should I do?

I have the same problem. I write scripts in Notepad. To avoid the hassle of specifying paths, I do the following: I click EditScript, quickly enter one letter (before the editor crashes), and exit. Then, in the map directory, I find the MpaScript.lua file and type there.

how to block cells near a garrison and then unlock them upon a certain event?

Create a region (the Regions tab is in the same row as Objects, Tiles, Masks). Set a trigger for the event (the Trigger function is described in the manual). Block or unlock the region using the SetRegionBlocked function (the description is also in the manual).

how to add random creatures level x to the Additional Stack in a mixed unit?

I don't know, I haven't done that. You can also use a script to set random values (the random function).

how to make quests be given one after another, not all at once?

This can be done without scripts. Each quest has a property called dependencies, where you specify the names of the quests that must be completed as a condition. There might be something else to configure (IsHidden or something like that, look at the meaning, and it's also in the theoretical guide).
If you find it difficult to understand the English manuals, remember that there are some in Russian in our file archive.