There's two possible testaments on ScotlandsPeople for one of the Mary's parents (Mary daughter of George Merrilees and Ann Richardson):
Merrylees, George; 10/3/1790; residing at Salton, son of John M., farmer at Killeith, Haddingtonshire; TD; Edinburgh Commissary Court; CC8/8/128
Merryless, George; 25/11/1789; residing at Salton, son of John M., farmer at Killeith, Haddingtonshire; TD; Edinburgh Commissary Court; CC8/8/128
Given the residence details and the reference numbers are the same, it's possible that it's two index entries for the same confirmation. However it's a Testament Dative (TD) which means there's no will, just an inventory. If you want to chance 10 credits (£2.50) you could download a copy and hope that one of the executors is Robert Howden, son-in-law, which would solve your problem. However, there's also a chance that the executor is Joe Bloggs, Solicitor!