Your information on the children of Wallace Mobley agrees with what I have found.
Wallace's wife was Martha Jones, daughter of Thadrick and Mary (Wallace)Jones.
Dorcas Sims was married to one or more Mobleys and was said to be the sister of Mary (Sims)Harvill who was the mother of the Susan Harvill who married Myatt.
Myatt is definitely with an M - not a W.He can be found in many land and census records.
The Middleton Mobley who was Wallace's father was born about 1740 and was executed about 1782 for being Loyalist in the revolution.
I don't know of an Issiah or Iasiah.There was an Isiah but his dates are later.You could probably get more help from Carl Mobley whose files are extensive.See http://freepages.genealogy.rootsweb.com/~mobleyhttp://freepages.genealogy.rootsweb.com/~mobley.