The Hero's Path

There will definitely be some losses in every battle on auto-battle.

No, there won't be :) - if you have the Cloak of Death and the Staff of the Underworld, the speed and initiative of enemy units are reduced, so having only archers and paladins in your army, you can destroy all enemies even on auto-battle before they start their turn, and the tent, if needed, revives those killed by the enemy hero (attacking spells are somehow not used by anyone except the Dark Elves). I'm attaching an example of such a battle (I didn't do anything manually).

Regarding the years of in-game time, I'll check tomorrow, but it seems that it took only a couple of months to complete the game (if measured in local days equal to weeks, or a little over a year in standard time).

As for the deployed units that have already been taken back, they don't disappear anywhere, they just stay in place, and if you click on them a second time, nothing happens, the cell remains blocked and you can't pass through it, as if there was some item there. The screenshot is attached - the unicorns have already been taken (there are none left), but they remain on the map, and it is impossible to pass by them or destroy them; when you attack them, a sand timer appears for a while, and then everything returns to its original state. They are not displayed in the notebook (second screenshot).
This has always happened to me, so I tried to deploy units either in places where they wouldn't interfere, or I simply banished them (if I was sure that they would no longer be needed).