First I should say, for the benefit of any who might read this, my personal interests are confined to the decendants of David Porterfield Sr. since they would be the only ones I am related to.
My observation concerning James Porterfield who married Catherine (?) Maxwell is with the age ranges. Robert Maxwell, Catherine's first husband, died in 1752 at the age of 39, making his birth about 1713-1714. If you estimate Catherine's birthdate based on her children, James Maxwell on or before 1739 and David Porterfield bet 1755-1760, you might reasonably assume Catherine's birth to be bet 1715-1725. If James Porterfield is in the same general age group as Catherine, which of course he wouldn't have to be. Then he would be in the same generation, as the son's of John Porterfield of Donagal Co. Ire.
1John Porterfield (1675-1739) 2 William Porterfield (1712-1789) moved to Berkley County, VA
2Charles Porterfield (1714-1778) moved to Frederick County, VA
2Robert Porterfield (1716-1785) of Hanover Township, Lancaster County, Pennsylvania [which became Dauphin County in 1785]
2James Porterfield (1718-1777) moved to Cumberland County, North Carolina [he and his descendants were ruled out in answer to question #1(a)]
2Patrick Porterfield (1720-1789) moved to Augusta County, VA
2John Porterfield (1724-1786) of Cumberland County, Pennsylvania
2Samuel Porterfield (1726-1789) of Cumberland County, Pennsylvania