If done properly many (not all) gliders could go from what looked like a normal attitude, normal airspeed, balanced turn to a fully developed spin, with the _only noticeable_ warning being "progressive rearward movement of the stick".
Not sure I understand that. The above description sounds like the spin was set up from 1g flight, so how could the IAS be desribed as normal. Surely the IAS must have been decreasing toward the basic stall speed. In that case the low and decreasing IAS would be pretty obvious indicator.
Obviously an aircraft can stall at a higher speed than the basic stall speed, but that would mean that g must be increased - again, another fairly obvious indicator when coupled with IAS.
Was never taught to note stick (elevator) position when stalling, and have never taught it. Furthermore I have never seen any reference to stick position in any instructor manual when dealing with stall recoveries.