Cliff, I'm not as sure as you are that Francis Fiveash is the ancestor of the brothers John and Peter Fiveash.As far as I know, the sole mention of Francis Fiveash is his appearance among the headights claimed by Thomas Grey in 1639 for his patent in what became Surry County.There is no further mention of anyone named Fiveash in Surry County or anywhere else for another 44 years.
I think it has to be significant that no Fiveash appears among the Surry tithables from 1669 through 1682.John Fiveash first appears as a tithable in 1683 and Peter Fiveash in 1686.Since both were clearly several years older than the minimum tithable age when they first appear, it means they came to Surry from some other location.
An alternative explanation is that John Fiveash is the same person claimed as a headright by Henry Randolph in 1691.I note that some of Randolph's headrights came from a 1685 certificate, meaning that some or all of them could have immigrated at about the same time John Fiveash first appears in 1683.I also note that several of the other headrights appear in both Surry and Isle of Wight records.
As you know the majority of immigrants were never claimed as headrights, so the absence of a name from headright lists is meaningless.
Also, the John "Hancoke" claimed as a headright by Grey could not have been an ancestor of John Hancock, the signer.His genealogy is pretty well documented and includes no one who set foot in Virginia.