As to regression and plateauing, I tell my students the following: focus on basics. It is likely you have become sloppy with them and the sloppiness is inhibiting your ability to move ahead. Ufortunately, everyone wants to do the "cool" fancy techniques. I try to integrate them in the training with the emphasis on doing good basics being necessary to make the cool stuff work. Recently, I did one hour on basics and 3 hours on kaeshiwaza. It was fun, although I may have spoiled them on looking for ways to get out of nikyo and kotegaeshi.
I haven't seen shodan regression syndrome yet. THe ones I have there now are as dedicated as one can expect. Perhaps this group I have in the Brown belt ranks will experience it. Don't know.