1) "Man with a metal arm" is dog-bullhorn for Tezzeret. Even in settings with conspicuous cybernetic augmentation, WotC's built up enough lore for the specific calling out of "a man with a metal arm" to not be anyone BUT Tezz.
Teferi: Better not let him hear you call him that.
Oh, what's he gonna do, fail at me? The ONLY time he's ever been successful at anything is when he stole the Planar Bridge, and that was because he'd had massive amounts of plot armor, and even then, it became a fail, because nicky-b bonded the Bridge to his body, leaving him a living portal frame for an army of TerminatorsLazotep Zombies. How much you wanna bet that nicky-b forced Tezzers to assume the Goatse position while serving as the living portal, for maximum failage?
Ral: Fought him on Alara after the War of the Spark; can confirm, the gaping hole wasn't in his chest! The censors wouldn't allow the real location to be revealed, if you'll pardon the pun.