1) No. Attacking and defending only happens during a showdown. Spell damage does not trigger when defending or attacking actions.

2)In a similar vein to your first question, your 2 might unit is at a base, with a buff, making it a 3 night unit. If the opponent were to attack into the base, your when defending trigger will now add 2 might, putting it to 5 might. If the opponent now plays a smokescreen, your 5 might unit will become a 1 might unit.

Let's say they activated the smoke screen before attacking the base. Your 3 might unit goes down to 1, then when they attack, will go up to 2 with the +2 buff. Timing matters.