Don't know if you'll find this helpful or whether it will just add to the confusion, but...The Robert Lumpkin who married John Hendrick's daughter was surely not the one born 1756.
On 25 Feb 1766, Hans Hendrick sold his 194a in Halifax County to a George Lumkin of Orange County, North Carolina, the deed being witnessed by George Lumpkin Jr. and Robert Lumpkin.[Halifax DB 6, p152]The following year, it is the same George Lumpkin who appears in the tithables with George Jr. and Robert.The Robert Lumpkin who witnessed the deed and appears as a tithable the following year is surely a decade ro so older than the one born 1756.
The land description used in the 1766 deed was twenty years old, being repeated from the survey of the original 1748 patent.By the time of the sale to Lumpkin, it was within a mile of the land upon which John Hendrick had settled, described in the later sale by his heirs as the land upon which he "lived and died."
I am unrelated to all of these families, and have not looked at further Lumpkin records beyond the Hendrick sale identifying Robert Lumpkin as an heir of John Hendrick.
I would note, though, that Robert Lumpkin, Charles Smith, Humphrey Hendrick, and Absolom Hendrick (4 of the 6 heirs) all were in Oglethorpe County, Georgia within a few years.
I have no idea which daughter of John Hendrick married Robert Lumkin.I understand his widow was named Drusilla, but whether she was his Hendick wife or a later one I don't know.