1) Is this a shield or another permanent (ex. Unstable Gas)? -another permanent, hence why i mentioned the effect cannot stack
2a) Does this apply to spells and permanents? -no, just when your opponents creature attempts to target yours
2b) If so, is the miss chance% decreased or increased for multi-targeting spells/abilities? n/a
3) The wording in the description gives the illusion that their attack may miss, rather than their ability, which I assume you are referring to. -it makes no mention of attacks, only abilities targeting your creatures.