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These vowel sounds don't shift to |a| though, especially as pronounced in Darlene. if anything Darlene may shift to Dorline (in accents where a is more of a back vowel like o) or Derline (leveling influenced by the i in the following syllable), but not the other way round. That's just not how sound-shifting works. Sound-shifting has directionality, and follows rules - levelling, breaking, diphthongisation, monophthongisation, raising and lowering (all under certain conditions). Some of the citations you give from the 18th C and earlier could plausibly be derived from Darlene, if Darlene were attested earlier, but Darlene cannot be derived from them.