Mara has a short 'a' to me, but that isn't very helpful as I'm not North American :-) Incidentally, there is no long 'ahh' sound in my accent, so it's a non-dilemma for me. A bit of a shame though, since the long 'ahh' is prettier I think. If I could, I'd pronounce it MAHR-ah.
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♥Elinor♥