I think my Henderson family is connected to yours. My great-great grandfather was William Henderson, married to Janet Mackay. I believe William was born C1821 and Janet born c1825 but not sure as I have no exact dates. William and Janet were married in Wick, in 1863.
In 1871, William and Janet were living in Wick - there were a number of Fergusons also living at the same address.
William's parents were David Henderson and Barbara Forbes. David was born c1797 and Barbara c1800.
Given the name of one of your David's daughters - Barbara Forbes Henderson born in 1868, could it be possible that your David had a brother named William?