I am also descended from Pyburns in the Northwest Florida / South Alabama area into the early 1800's and possibly earlier.I have been trying to connect my line with your Pheobe Pyburn Turvin, but have had no success.
The Spanish land grant that I am familiar with involves land in Pensacola, Escambia Co, FL, 1817.It mentions 1600 arpents of land, cattle ranching, and a brother named Santiago (St. James?).Is this grant the same as yours?
There is a Fabia Paiben,widow,listed on a Spanish census record of 1820 Pensacola.She lived in a Collins household.I don't think it is a stretch to say this could be Pheobe.