Mariah was the old pronunciation of Maria before Americans realized how Maria was supposed to be pronounced. Using long I's where it's supposed to be "ee" is something that happened a lot in "the olden days" but doesn't happen very much now. This is why Elvira is pronounced El-VIE-ra to you (because you haven't heard it otherwise), but why BtN says it's pronounced El-VEER-a. And this is also probably why =A='s family pronounces Elmira "MIE-ra."
Now that everything is globalized and the pronunciation rules of Latinized names are more well-known, Elmira is generally pronounced "elmeera," as the Correct Way.
*shrug*
Silly debate