In order to simplify the extruder path, today I used a different piece of PTFE tubing to run the filament straight from the extruder into the hotend (replacing the usual hotend PTFE liner). This gave me a much more direct path from extruder to heat block. The result (no photos - sorry) was much improved with the NinjaFlex being extruded far more reliably. Not *quite* print quality yet, but a little fussing with temperatures and extruder speed might be enough to fix that. This gives me hope that, if I can straighten the pathway for the flexible material, this still might work.