I have been looking through Hiski and i found today the wedding record of Margreta and Petter Hynynen (1714- 1792).
5.8.1753 Jumis Petter Hem.son Hynyin Pig. Marg. P.d:r Heiskain. Korpijerwi
It is definitely our couple but this is the only place that I can find Margaret's middle name (patronym). Whether "P.d:r" is short for "Petersdotter" or "Paulsdotter" I have not yet found but certainly Jeremiah does not seem to be her father as we have shown on MyHeritage and Geni.
Absolutely no sign of anyone named Margreta Heiskainen ever being born anywhere.
Luckily Margreta was not a very popular name in 1726!
It is just possible that in reading the original hand writing the "S" changed into a "C"?
11.11.1727 Pehr Jacobss. Heickinen Chirstin Andersd:r Margareta