For the Queen!

1. I think the experience level caps could be raised. All four heroes who can advance to the next scenario reach level 6 very quickly.

asseco, welcome to the hard version ;)

Level-up boosters (trees) are not needed.

The "Temple of Knowledge" (or whatever it's called) is useless – none of the heroes can use it because they "don't have enough knowledge."

I won't change that – after all, these are already in-game objects ;) :)

3. Releasing a hero from prison could be made mandatory, for added interest.

I'll look into what can be done about this specifically for the hard version.

Yes, in the second scenario, 4 heroes successfully advanced to the next level, all at level 6, without any errors.

That's because in the normal version, I set the transition, but I forgot to do it in the hard version.
+
regarding the hero in the dungeon:
a hero can be captured by either the player or the computer. We could create a script like this: capture the hero, and it must be a second mandatory condition that the hero is alive (after capturing the city), and if the computer captures the hero, then the mandatory condition is to destroy that hero. :)
I'm not a scriptwriter myself, so I'll ask Ser9K