If an enemy Hero is under the effect of Chill Aura which reduces the result of their dice roll to win a fight, can that enemy Hero then use a Might point to bump up their dice roll again?

If the enemy Hero rolled a 6 in battle, Chill Aura would cause that roll to count as a 5 towards winning the battle.

Does Might affect the final value, or just what the dice would have rolled (max 6)?

It's the same rule as rules for 2 handed weapons and in that rules they say you may use might to make it to a 6.

So does that mean, if I roll a 6 and my enemy rolls a 6, he can't use any Might to affect the dice roll, and that the final verdict would count as a 5 from the Chill Aura?

you can make a 6 of it read my other answer.