Bottom line, we can read Melech Shlomo, but might not understand him if meeting in person… Of course, everyone thinks he will because he surely speaks the genuine version of loshon kodesh …
And, also, how is Eliahu communicating with tzadikim – does he speak a different loshon with each of them?
On a more pragmatic note – is there research in how old Hebrew sounded? I heard simulations of old English … A simple thought would be that a version that differentiates between more sounds is more likely to be genuine, such as Teimani ayn, and patach/kamatz, but I am sure linguists have more to say.