I wonder if it didn't start out as Tomina. Medieval handwriting being what it was - right u p to and including the 18th century, actually - it could easily have done what Amabel famously did, but in reverse. Tomina -> Tomima, and Amabel -> Annabel.
And, yes, the Jemima connection could lend it some appearance of authenticity, just as Anne did for Annabel. I'd also guess that it'd sound like to-MIE-ma nowadays, both from the Jemima connection and because the Scots have a traditional affection for feminising names like Andrew and Donald by affixing an -ina and making it sound like IEna.
Wherever did you find it? It's certainly new to me. Well spotted!