I hope this is the only reply. After I sent you a reply, I received a message it didn't go throught.
Mari did not die in 1791. I don't know when she was born, but I don't think she was born before then. My guess is she was born closer to the turn of the century.
Niels was born 19 Jun 1826. See his baptism record attached to my first request in this thread. Lorenz married Anna Margrethe Lund 30 Dec 1796. Her first child was born in Farsund in 28 December 1797. Lorenz was in his 60s when Niels was born and he died just prior to Niels' 4th birthday. Anna didn't die until 1844.
The math does not work. Also according the baptism record, Mari was unmarried at the time of Niels' birth. He was her 3rd child born out of wedlock.