My memories of GM Agrella go back 40 years, in those days he was often running the tournamnts. Even then he was a Grand Master, everyone accepted that. His students did long powerful forms and were among the best fighters in the old Region 10 (NJ, Pa, MD).
Haven't seen them in 36 years living now in NH.
As to was his preying mantis legitimate, not the Mantis I studied or like other Chinese Mantis systems I've seen. But everyone seemed to accept it and his students were dominate then. I recall him telling me he trained with Oyama too.
There is a abundance of systems.
bushi no te isshinryu
offering free instruction for 30 years