Yes I think you're on the right track Neil!
I found the 1840-marriage, Johan is listed as "Soldier" in the "Klokkerbok". If he was alive in 1875 it is quite likely he died in Fredrikstad some time between Jan 1, 1866 and Dec 3, 1900.
In the 1840 marriage the bride's father is Herman Monsen, in case you haven't got it already...
J