"Mary" could also be Polly, Molly, Moll, Marie, May, in fact when a name's in doubt it seems to have been an affectionate shortening of Mary, in the families I've researched. "Margaret" in my mobs resulted in Mags, Meg, Maggie, Margery, Mads, Meggie.....
"Elizabeth/Elisabeth" seemed to morph into Liz, Liza, Eliza, Lizzie, Elspeth, Isabell (which I'd always thought of was always a distinct name on its own, until I found it being used almost alternately on censuses for the same lassie, Beth, Bettie, Betty, Bess, Bessie ....
A confusion I had, one ancestor I'd thought I knew the name of her, until I saw an official document, "Cis", I'd assumed was short for Cecily or Cecilia - it turned out to be "Cis - for Sister", as the oldest of a brood, that was how she was known to the entire family. No wonder I'd not been able to find her birth. She turned out to be "Annie Elizabeth".