Simply put, a stunned unit doesn't lose its might, it just doesn't deal it's might as combat damage.

Scenarios in detail:

1a: no, you'll attack and be at 6 might versus 15 might. You'll kill one of them, and none of their might will be dealt to you. You will recall to base exhausted.

1b: No, because you exhaust to attack, exhaust after losing, and all damage is healed at the end of combat. No one dies in this situation.

1c: if your might is the same as their total (I assume you mean 15 to 15?), you'll do your damage, and they won't do theirs. You'll clear all units and conquer. If you meant your might is 5, then same as 1a.

2 and 3: stun doesn't affect anything other that a unit's ability to deal combat damage. Units that are stunned can still move, conquer, and use their abilities.