Blake did make reference to a different type of shoulder turn in GTTB which he likened to that of skimming a stone across water. He wrote: "...the shoulder turn involved in skimming a stone across water--is very close to the way the body works in the swing I am about to describe." In his description of setting up for skimming a stone he said, "...the shoulders are slightly open and the pelvis is thrust forward." About the skimming action itself he wrote, "The shoulders, which started to turn with the hips, continue to turn after the hips have stopped." Later, in the Backswing section: "The shoulders complete a 90-degree turn from square after the hips have stopped and the movement is the same as the stone-skimmer's turn, a simple athletic movement." These words do not suggest a limited shoulder turn, however, I agree that the drawing in figure 15 does not seem to depict a full 90-degree shoulder turn, but in the words acccompanying figure 15 he wrote, "At the top of the swing I feel I have swung the trunk and arms as far to the right as I can, using the legs as a counterbalance." Swinging the trunk and arms as far to the right "as I can" would seem to produce a full shoulder turn, especially by a person of Blake's suppleness. In my own instinctive way of skimming a stone, my inclination is to "whip" my right arm, rather than "drag" with the legs, as I come down from the top to let the stone fly.
As to whether a limited shoulder turn is a key aspect of the reflex swing, I surmise that Blake would see the trail elbow position and use of the legs as the sole source of power to be the key aspects. However, with GTTB and his adoption of a Lee Trevino-style open stance, he thought he had found a solution to the problem of a good hip-shoulder relationship at address such that the hips will not turn with the shoulders and "collapse." By starting with the hips open, related to the shoulders, the hips can only turn back to about square at the top. The "super results" you have been getting with a more limited shoulder turn is intriguing. Do you find that your backswing is shorter with the more limited shoulder turn? Please keep us posted on your progress.
How did you make contact with Roger Judd in 1987, before the days of email, Internet and such? I notice in Harry Weaver's introduction the following: "He [Blake] also received a number of requests for a home movie of his method." Could Roger's film have been a partial answer to that request? Weaver also mentioned a golfer from the Midlands who drove 300 miles every Friday for several months to spend a half hour hitting balls under Blake's critical eye. Might this have been Roger Judd?