Not being in the US, I really can't predict what people might think of it.
What I thought was that Mattie or Tilly would be good nns for Matilda, and Mimi or Jimsie for Jemima; Lucinda gives Lucy and Cindy of course, so if it was me I'd play the nn game, combining them with the ln, and choose the entire name accordingly.
And, of course, you're right. Once people know a pleasant, friendly Jemima person without obvious hangups, their personal associations will change.
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