Walkthroughs for custom maps.

Is this an acceptable bug or a legitimate feature, similar to boarding a boat with slow units and then removing them?

Everyone decides for themselves how and according to what rules to play in single-player. Some don't hesitate to fire and hire heroes on the same day, restoring movement points and mana. Some use selling artifacts at inflated prices. Some use the Admiral's Hat, etc. There are no standards. The mapmaker should oversee this.

Personally, I always play according to the rules of offline tournaments, even for fun playthroughs. If you can't use the Admiral's Hat in a combined form, then you can't. I don't even like the idea of using this artifact in a combined form. But then again, there are some maps in which the Admiral's Hat plays a key role. Well, each person has to decide for themselves.