The May 1918 bout with Gunboat Smith was a five round exhibition, not a six round ND bout.
On June 20, 1919 you have Greb facing Pat (Young) Gilbert. This is completely wrong. Greb faced Yankee Gilbert on this date. Yankee Gilbert and Pat Gilbert were two different people. Yankee Gilbert should also not be confused with Gilbert Yankee, a featherweight of Cleveland who acted as Johnny Kilbanes sparring partner.
The two fights with Chuck Wiggins for May and June of 1921 in South Bend and Terre Haute should be newspaper wins for Greb. I have the ringside accounts of those fights. You list the Indianapolis Star as your source, Chuck Wiggins hometown paper.
Technically the January 1925 bout with Bob Sage in Detroit WAS a championship bout. In that era the title was generally accepted as being on the line provided the challenger weigh in at or under the championship poundage regardless of what the champion weighed in. This is true of ND bouts as well the difference being the challenger could only win the title via KO or DQ (DQ not generally being accepted though due to the referees interference in such a decision, this was the case in the Kid Williams-Johnny Ertle bout).