Current questions and answers regarding the map editor.

How can I remove the "highlight" around creatures? I mean, the circles under neutral creatures.

You can only remove the circles from disabled monsters. That is, the hero will not interact with them.

SetObjectEnabled("name", nil)
sleep()
SetMonsterSelectionType("name", 0)

And how can I turn a neutral creature into a creature belonging to player 1?

It is possible to simulate this using a lot of scripts and caravans. I've already done this, and the result was quite decent. If anyone is interested, I will post the script. But it will need to be adapted to the map.

Added 7 minutes later
I will post screenshots now.