Tom MacWood,
I believe you are minimizing the impact a course like Merion had on Flynn when you say rather dismissively that "one course design (or redesign in this case, in collaboration with another)..."
Flynn was at Merion working on that course (and the West course) for five years, and was extremely instrumental in helping shape and implement the changes that were implemented for the course to get it ready for the 1916 US Amateur. Contemporaneous news articles of the time mentioned exactly how lucky the club was to have a guy as knowledgeable as Flynn.
He basically spent five years at Merion as training for his career as an architect, working at Hugh Wilson's side.
This had to be incredibly influential.
It wasn't until around 1916 that Flynn started doing original designs (not counting Hartwellsville), after five years at Merion. (correct me if that is incorrect, but that's what I'm recalling).
He then worked with Wilson for the next 8 years, implementing a number of other changes for the 1924 amateur, including the extensive re-routing that took place for holes 10, 11, 12, & 13.
If a dozen years working with Hugh Wilson wasn't the most influential part of his formative career, I'm not sure what would be?
Of course, as Wayne points out, Flynn came back for another decade after that and did additional changes to the Merion course on his own.