You have an interesting story to tell and it is with regret that I have no info. to offer you to help unravel your problem.I know the feeling of frustration as I have my Moncrieff's back to 1758 but can go back no further. There is someone else who has an extensive Cellardyke/Kilrenny Moncrieff lineage (posted on WorldConnect)and I am convinced mine are related to his, but I'm not able to tie them together and he's not interested!
Anyway, have you done any research on James Wellwood Moncrieff?Surely you have but if not, that could help. Have you researched under Alexander John Moncrieff besides under Alexander John Robertson?
Several years ago I used a Scottish researcher since I’m in the US.If you are interested, write me at firstname.lastname@example.org and I’ll give you her e-mail address and regular address.I’m not sure if she has a website nor if she is still in business.
By the way, I would not be surprised that at some point the spelling of Moncrieff was different, and probably Moncreiffe if from the Perth area which seems to be the case.Have you seen the huge Moncreiffe lineage on line?Right now I can’t get into it but if you don’t have that, I’ll send the address as soon as I can get in.
Best of luck to another Moncrieff; a beautiful name of which I am proud.