Unfortunately, Beta is closed now, so I no longer have any way to test An-Windr.
But the card answers your first question, and @DaveCozy mostly answered the second.
They’re separate increases, and the attack buff appears to apply to the Special Skill too.
The card reads:
So there’s a damage increase on his attack, and also an increase on his Special Skill damage.
Here are examples of the Boss version dealing massive damage:
@DaveCozy indicates they stack in 📑 Status Effects (Buffs and Ailments) and Stacks - Dead topic see link in post #190
I’m not sure about the cap, though I would guess it might be the same as Berserk, which tops out at 144% (i.e. 5 hits).