"Response: After you play Erebor Hammersmith, return the topmost attachment in any player's discard pile to his hand".
Can I use this effect for more than 1 player? Like me and the player B? Or just 1 player?
Well there still are two ways to read this. "Topmost" is ambiguous. Topmost in the pile you choose? Or topmost over all piles.
To be more clear, here are the cases:
a) Choose a discard pile: return the topmost card in that discard pile.
b) Among all of the discard piles, whichever has an attachment that is highest (closest to the top), return that card.
I believe a) is the intended ruling.