Good questions

"What does it mean that the troops were in the full view?"

On both occasions I had my units deployed so that I could see the enemy units. The terrain on the first was Mountainous. the enemy was on a slope across the valley and i had taken a position opposite also on a slope.

"What happened to the units that the enemy did react to, where did they go, did you retreat them toward your main army?"
I retreated towards the main army and the enemy stopped showing any interest in them

The minimap did not change when we engaged

I have no screen shots (will try next time) I think I can
make it happen by:

Starting a battle and allowing the first 16 units to get routed, moving the reinforcement flag to a completely different part of the battlefield and bringing on reinforcements once the enemy has settled into position.

However in the firstinstance I had troops left from the intial deployment and simply marched them away from the site of the first battle and the ai did not follow.