[Facts] Re: Muriel / Mariel and English dialects
I would not say that Mariel is a variant spelling of Muriel. Here they're considered two distinct names. Muriel is MYUR-ee-el (first syllable rhymes with "pure.") This was my aunt's name and she was from the Midwest, but I live on the West Coast and it's pronounced the same here.
Mariel is MARE-ee-el (first syllable rhymes with "care.")