Re: John Corbin (b c1698 VA - d c1758 VA)
There is no date other than the probate date (November 2, 1758) on the will of John Corbin of King George Co., VA. For a person of his economic status, it would be customary to have a probate of the estate within a few months of the time of death. Given that the inventory and appraisement of the estate wasn't returned to court until May 1759, I think that it is reasonable to assume that this John Corbin died about September-October 1758.
There is no "evidence" that I am aware of that the John Corbin who married Elizabeth Smith was born circa 1671. This is just family tradition and possibly the Corbin Diaries, both of which should be considered as questionable sources for this early period. The property transactions to a John Corbin of Richmond Co., VA in 1692 and 1696 (#1 and #3 on your list) were almost certainly to the man who married Elizabeth Smith. There is no evidence that the father of this man was ever in Virginia, simply family tradition.