How would you translate it using "in"?

I thought it was an instrumental dative also, but Fagles, Lattimore, and Smyth all translate it as if there is either an understood "epi" or as if there is some separation implied, i.e., " exemption from our concerns." Hence the μελέταις are not the means by which the exemption is effected; rather, the exemption is from the μελέταις, which is why I am confused why it is not in the genitive case.

There are things implied here, because even if you translate it as an instrumental dative, you will still have to explain what is achieved, ensured, etc. by these μελέταις.

So you are saying that simultaneously the concerns of the Furies are both bringing about an exemption for the gods and causing the gods to be exempt from their concerns?

Exactement.