I am not a direct descendant of Gross Robinson and Sarah Anderson, but I am related through both the Robinson and Anderson families. My line from John and Tabitha is though Edward Robinson Sr to Edward Jr, to Judith who married Thomas Shirley Anderson Sr (brother of Sarah Anderson who married Gross Robinson).
I am confused as to why you think that Obedience could not have been a child of Gross and Sarah simply because she married in Cumberland Co., VA in 1838 and is listed as Obedience B Robertson. Lucy married Jackson G Meador in Cumberland Co., VA 21 Dec 1840, yet she is accepted as a child of Gross and Sarah. This marriage record lists her as Lucy Robertson. Also the marriage record of John B Criddle in Cumberland Co., Va on 22 Aug 1831 shows the name as Catherine Robertson yet she is accepted as a child of Gross and Sarah.
On 24 Sep 1847, Zachariah T. Talley bought from Sally Robinson, Elizabeth Robinson, Nancy Robinson, Emily Robinson, Frances Robinson, Ann Robinson, Henrietta Robinson, Jackson G. Meador and wife Lucy, and Gross Robinson per interest of William A. Robinson and Thomas Meador and Thomas's wife Biddy, 109 acres for $327. This land was on the waters of Deep Creek and part of the estate of Gross Robinson, dec'd. (Cumberland Co., VA, Bk 26, p 434. LDS reel 30757)
Notice that neither Ellen nor Catherine are listed in this deed yet are accepted as children of Gross and Sally. Why would Thomas L Meador and his wife Biddy have a share and interest in Gross's estate if Biddy were not a child of his?
In the estate records of William Anderson of Cumberland Co.,VA Sally is listed as Sally Robinson in a portion of the records and later as Sally Robertson and later still as Sally Roberson. In 1835 her signature as Sally Robertson as a legatee of William Anderson was witnessed by Creed Haskins.
In 1767 the will of John Roberson was written listing all his children as Roberson. Testees to the will included Roberson, Robertson, and Robinson. The will is written as Roberson yet the appraisal of the estate in 1768 is Robinson. These spellings were as interchangeable back then as they seem to be now.
The earliest record I can find for Sarah Anderson Robinson in TN is 1850 census of Jackson Co. Do you have some documentation of her and the children in TN prior to this time? Both Lucy and Obedience were in TN by 1850 as well.
I do not claim to be an expert on this family, but I do have a great deal of records and information on them. I am aware that there are many posts and tree listings on both Robinson and Anderson families out there that are horribly incorrect. You stated that Gross's children are well documented, and I would love to be able to share and compare this information with what I have found over the years. Thanks