I'm not sure if this helps but let me give you a couple of quotes and then I'll tell you their source
"William Pannill, the 4th, born at "Green Level", January 31 1768. He moved to Oxford North Carolina and there he married Martha Mitchell in 1793 (widow of Soloman Walker) daughter of David Mitchell and Hannah---of Lunenburg County North Carolina.Martha Mitchell was born in Granville County N.C. December 12, 1762, and died May 18, 1842.William Pannell was killed in Kentucky by a falling tree during storm.He had gone to Kentucky to survey land that his father left him."
"William Pannill the 4th born at 'Green Level' January 31 1768 son of William the 3rd and Ann Morton.He married Martha Mitchell. Their children are 1. Col. William Pannill the 5th 2. Elizabeth Davis Pannill"
These quotes are from a manuscript done by Miss Mary Stuart Fitzhugh who was a direct descendant of Thomas and Katherine Pannell, she was born 1881 and died 1967 in VA lived mostly in Orange Co.The manuscript is in the possession of LDS in Salt Lake City, UT, a former correspondent of mine now deceased furnished me with a copy.If you contact them or have an LDS church near you they might have it on microfilm now.It is a little hard to follow as there is no index and she would take entire family line of one child then pages later go back to the one following.I went back and looked through but these were the only items I found about the William you asked about.
Miss Fitzhugh knew a great deal of the family history down to her own time but did not know all the information of every descendant, and the two children above are the only ones she had listed.She did write their lineages but since neither of them is John, I have not included their children.
Green Level was the name of their home place in Orange Co., VA.
If you do connect to this family you will be able to trace directly to Katherine and Thomas Pannell of Old Rappahannock Co. VA through their son William Pannell the first. I may have some other old documentation somewhere and if I do I'll get back to you.
I don't know much about the Alabama Pannells and at this point I can't look up census records as I have temporarily dropped my Ancestry and Genealogy.com subscriptions.I had to stop research and get my papers organized a little better so that I could make sense of it all.
I'm curious about the two references you made to the two children of John in 1880, one in MS and one in AL.What were their names in case I run across them sometime?
You probably notice that I have used both spellings.In the documents I have looked at, except for misspelled words, the early ending was "ell" but the branch from the 3rd William changed it to "ill".
Good luck and write if you need me.I may not know the answer but I do have a lot of miscellaneous data I've accumulated over the years.