Regarding the rules about summoning:
As long as the summoned monster is present, no effect can target the enemy that summoned it. The summoned monster can be targeted normally.So, IMO, Nature's Vengeance cannot target summoning enemies, but they can target summoned enemies.
Scenario 1: You would have to play the skill versus the brown monster. It gains -1 to one of its attacks and cumbersome.
Scenario 2: You would have to play the skill versus one brown monster. It gains -1 to one of its attacks and cumbersome, the other enemy is unaffected.