I know with as much research as you have done on this family you probably have the following information, but just in case I thought I would share it.
From Frederick County Marriages 1771-1825
Murdock, William and Elizabeth Read, June 9, 1796 A.B.
Murdock, George W. and Jacqueline H. Smith November 10, 1815 J.W.
McPherson, John and Jemimah Murdock, July 6, 1789 A.B.
McPherson, Stephen and Phoebe Murdick, December 11, 1794 A.B.
I gathered this information over 12 years ago when I first started doing genealogy.I did not have the good sense to get the name of the author or compiler or the copyright dates, just the name of the book.The names meant nothing to me back then either as I was fairly certain they were not my direct line.I cannot tell you what the letters after the dates mean - there is nothing in my notes indicating what they meant.
I just checked the DAR Library Catalog and it shows: Title: Frederick County, Virginia, marriages, 1771-1825 / compiled and edited by Eliza Timberlake Davis Author: Davis, Eliza Timberlake. Imprint: Baltimore : Clearfield, 1996. Physical Description: 129 p ; 22 cm. Note: Index Note: Orig. 1941 ed., A#31411, with duplicate books.
I can tell you that George W. Murdock is George William Murdock, son of George Murdock (1742-1805), Register of Wills for Frederick County, Maryland as shown in
Newspaper Abstracts of Frederick County1816 - 1819 by F. Edward Wright on
Page 67 "198. 29 May 1819 ... Died Wednesday night, George W. Murdock, Esq., Attorney at Law, in his 27th year."(This means he would have been born about 1792.)
On Page 148 there is conflicting information: "410. 29 May, 1819 ... Died Wednesday night last, George William Murdoch, Ewq.[sic], only son of late George Murdoch, dec'd, in his 32nd year, leaving wife and 2 infant children, mother and sisters."(This means he would have been born about 1787.)
His mother was Eleanor Charlton (1747-1828.)
Sisters: Ann L. Harriet Eleanor Mary C.
Wife: Jacqueline Haines Smith
Children: George Potts Murdock John Smith Murdock
Also, I have found a John McPherson in Frederick County, Maryland.He was conveyed land by an Adam Keller in 1795.See http://ftp.rootsweb.com/pub/usgenweb/md/frederick/land/keller-mcpherson.txthttp://ftp.rootsweb.com/pub/usgenweb/md/frederick/land/keller-mcpherson.txt If you could find out when he sold the property, you may be able to confirm if he is the same John McPherson who married Jemimah Murdock.