I wouldn't make iron weapons
too hard to make. Lycanthropes can be taken down by silvered weapons, and silver's pretty common. Making a silver weapon is costly, yes, but not super-difficult. Fey-killing iron should be about the same. (Also demon-killing iron; they used to share that weakness in earlier editions.)
Perhaps the simplest way to go about it is to just add something to the metal as its forged.
@Yaarel suggested that Feywild and Shadowfell are parts of the Etheral. Perhaps... add ether?