Yes, you are correct, there is a dearth of testers from the Craskie and Duldreggan areas of Glenmoriston. I know of only one male descendant that has taken the test, this from the FamilyTreeDNA Grant Surname project:
"Group A (Chiefly), Subgroup 1:R-P312>DF19>DF88>Z17274>Z21133+ (Fruechie, Corriemonie, Shuglie, Glenmoriston, Blairfindy & Kilgraston)....
...Kit 373375 - is tied to Lewis Grey Grant born circa 1767 in Craskie,Glenmoriston,Inverness, Scotland.
Self
Henry W. Grant – born 1920; died 2012
George A. Grant - born 1889 in Kenyon Township,Glengarry County, ON; died Ottawa, ON 1961
John A. Grant - born 1842 in Kenyon Township,Glengarry County, ON; died Sudbury, ON 1917
Angus Grant - born 1813 in CharlottenburghTownship, Glengarry County, ON; died Kenyon Township 1886
Lewis Grey Grant - born 1767 in Craskie,Glenmoriston, Inverness, Scotland; died Charlottenburgh 1846
Donald(Craskie) Grant Sr. - born in Craskie, Glenmoriston, Inverness,Scotland; died Charlottenburgh"
That is all we have to this point that I know of. My prior statements were in regards to the minimal number of participants and DNA mutations necessary to prove/disprove the the Donald Grant lineage being from Craskie or Duldreggan.
Re: John the Tutor, I mis-spoke, soo many Johns and 111 degrees here today. As you know, John the Tutor was one of the three sons of John, 3rd OF GM. John the 3rd being the father of 1) Patrick IV OF GM, 2) John the Tutor - progenitor of the Coneachen Craskie lineage and 3) your Duncan - progenitor of the Duldreggan lineage, all noted in the Chiefs of Grant Book. This has been corrected in the prior post.
So the addition of this deeper ancestor would increase the testing range by a generation, hypothetically allowing for a bit better chance of capturing an SNP mutation for our hypothetical DNA test.
Now as to whether the Donald Grant b1744 Craskie flowed from either the Glenmoriston Grants, Duldreggan Grants, Craskie Grants or other male descendant from the Chiefly lineage, this is still unknown.
All that we do know is that he shares the same Y chromosome with all of them and there appears to be a number of possible paths for Donald to find himself in Craskie, including having been born from the Craskie Grant lineage(s).
If you are aware of any Duldreggan male descendants, please encourage them to take the test to help offer a better vision of the past.