I have been working on decomposing the genealogical hybrid John Jarvis for some time now. Most of the work online, and also the comprehensive work by Don Norman, have incorrectly combined 2 John Jarvis into one. As you have outlined, the John Jarvis who md. Ann Richards [no mention of her being named Sarah] md. in Maryland and apparently remained in Maryland throughout his life. They had five children, and only five children, and she survived him. He did have Rev. War service qualifying her for a widow's pension.
The John Jarvis who reportedly md. Sarah Hood O'Brien was a different person, poss. dates 1757-1823 as noted on his modern day grave marker. His widow Sarah is likely the Sarah Jervice in the 1830 census of Lewis Co., VA. He was apparently the brother of Solomon Jarvis who went to KY. They both reported unusual military service on naval galleys under a Capt./Commodore Mathews, also both mentioning a John Monhon/Monahan.
Please refer to Conley Cottrell Family Tree in ancestry.com and story "Military Service..." under either John Jarvis for further details.
Hannah Jarvis, mother of Solomon, attested to his War Service in Fleming Co. KY in 1829. She was living in Fleming Co. and was said to be in her 99th year at the time. She is identified as Solomon's mother in his Rejected Pension File.
Solomon attests that he enlisted in Capt. Mathews' unit in Belair, MD.
So, it seems that Hannah [poss Hill] Jarvis and John Jarvis who were in BelAir MD abt. 1779 are the parents of Solomon Jarvis who went to KY, and John Jarvis who went to Harrison and Lewis Counties, VA. Haven't tried proving this yet beyond identifying Solomon's mother as Hannah Jarvis.
Neither the John Jarvis who reportedly md. Sarah Hood O'Brien; nor his brother Solomon who went to KY; received anything - land, pension, anything - for their service during the Rev. War period. The reason is outlined in Solomon's rejected pension file - his service in the naval galleys under Capt. Mathews was not service to the Continental Line. The service that he attested to would have been in the Virginia Navy; and as such, even if proven, would not qualify him for any federal benefits.
So, the modern day grave marker for John Jarvis in Westin, Lewis Co., WV is in error, as it credits him with service to the Continental Line.
I assume the Thomas you are trying to trace is the Thomas Figgens Jarvis, attributed to John Jarvis and Sarah Hood O'Brien Jarvis. He reportedly md. Ailsa Bentley Conley?
It does seem most likely that he belongs to this couple, but I've not found any proof.
Hope this helps a bit. If you have any documentation to add, I'd be delighted to see it.