leaving a hero in a garrison – theoretically, yes.
I don’t know about a port city, but the possibility of attacking a city from the water without losing a turn for landing was recently actively discussed.
regarding garrisons – there are plans to improve them to the level of more serious defensive structures. this is not difficult to implement, even in terms of graphics – only one "neutral" wall template with damage is needed. this idea has been added to the list of the most successful ones, so – if we have time, we will do it.
the third level of the map is practically impossible.

Wow, I already forgot about the release of this masterpiece, and it is still under development.

fans of the Heroes series are used to long waits; if you want to get a quality product, you need to be able to wait.