HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

This is changing, see the first post.
Two 'Check' buttons in Swap Manager = 0

Regarding the quick transfer of troops, it's natural that clicking on the arrow under the unit will leave that same unit with the hero.

You need to pay a little more attention.

The focus changes only when the second button is disabled, but it would be better if the second button remained, not for nothing was it invented, with a "correct", i.e., cross-focus, both buttons could be used.

Clicking on the arrow under the unit leaves only one unit if the hero has no other stacks. The suggestion is to transfer the ENTIRE army with ONE click, simultaneously selecting the remaining unit.