Israel VanDyke is found living with his widowed sister Martha McGuire in Audrain MO 1880. Martha was married to Harvey Wesley McGuire, the first son of William McGuire and Nancy Mariah Harman. Israel's parents were John VanDyke and Nancy Mitchell.
As you know there is a brother of William named Joshua in Lt. William's 1836 will. He is also mentioned in 1837 in a court order, so if he died in 1838, this could be him based on the dates. I don't have the data to distinguish him from the older Josiah who was a brother of Lt. William.
I believe Mark T. Lockhart was a son of Bird Lockhart whose son John married Sarah McGuire daughter of Joseph McGuire and Elizabeth Brown.
Thanks so much for digging up and posting all these details.