Walkthroughs for custom maps.

It's easy to imagine a scenario where hiring a specific hero is permissible later on, but at the start, it would ruin the author's design.

In Heroes 3, there's no mechanism that prevents a hero from becoming available to a player one week but allows them to be hired the next. :confused:
Let's consider the opposite situation. The map's design requires hiring a specific hero (e.g., Gem for summoning a boat), but due to some circumstances, I can't find him. What then?
I believe that if a hero is available for recruitment and isn't already hired by another player in the tavern, then I should be able to find him (except in cases where two identical classes are located nearby). And that's it.

As for the time spent manually searching for the desired hero, it's a small amount compared to the total time spent completing truly challenging maps.

Maps vary. You can't equate all maps to Chinese-style maps. Some maps are inherently difficult to complete with any outcome, while others are easy to complete, but achieving a maximum speedrun is very difficult. In those cases, the competition comes down to millimeters, and searching for heroes is just a routine task that must be done and cannot be avoided.