1. I think they are out for revenge because of what Marcone has done. He has overreached and they will punish him for it. But I also think he will be a key ally in the fight against them.
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3. Gard describes the Fomor as being shape-changers, sorcerers and seers. I suspect more than one Fomor has or will impersonate someone (the shape-changing thing) and Harry will fall for it. If not others too. Beings seers, they must have some ability to predict the actions of others. This might make them more challenging for Harry than normal.
1. It seems they hate mortals position in general and Marcone's specifically. I'd call that prediction confirmed.
3. Perhaps Thomas didn't attack the Svartalves at all?