Scripts

Regarding the configuration of reserve heroes available to any player, it can only be done through a script. However, I recommend trying to add the same hero to the reserve for all players. To do this, in the Map Properties Tree => players => [0] => ReserveHeroes => copy the AdvMapHero link to the reserved hero and, for other players in ReserveHeroes, add a new element (Insert when the ReserveHeroes property is selected or right-click -> Add), and copy the link into the new element. I haven't tested this method, but it might work. It may cause crashes.
You can remove a hero from the reserve using the UnreserveHero command.