Hello!
First of all, happy holidays to you.
I wanted to let you know that I figured out your method, it's very simple. Thank you for the hint.
HOWEVER, I want to say that you might be mistaken - the mod you recommended works on its own with downloaded maps without any additional intervention. Yesterday, I loaded three randomly selected maps that I downloaded from this site, and all the heroes from the mod were available there. Moreover, I checked in the editor whether the checkboxes are set next to the heroes, as you said, and on some maps where the checkboxes were not set, all the heroes were still available.
If any questions arise, I will write again.