Magic Duel and communion

[Corwin]

Do I remember correctly that bonus levels from Communion counts when communion Master cast Magic Duel?

Also is there any rule regarding which of the enemy astral mage would be challenged to the duel? Would it be the enemy astral mage with highest level of astral, or is it random?

Bonus question - what happens to communion masters when their slaves retreat (or when they are killed). Do they reverse back to the original magic levels?

And what would happen to communion slaves if their master retreat from battle? Will they keep their personal buff spells that they got because their master casted it while he was still around, or would they lose it?