CVV, even within the Higa (Shodokan) line: (Shoreikan, Senbukan, Kenshinkai, Kodokan) there are differences, but as you and Victor mention, follow the same pattern. At risk of sounding like I'm stating the obvious, I suspect the differences are in the interpretation of them...since everyone has more or less their own. and if you are imagining a certain application when training, the movements are bound to be slightly different when you teach them.

but I think looking at the Higa line doesn't say for sure what Higaonna taught since there may have been collaboration to 'standardize' Seisan in Goju after Miyagi's death. I'm not certain though.

I finished reading the To'on-ryu articles, M. McKenna always does some great work. I've never seen that type of stomp in Goju. Seems like that application could work if you have hold of both the opponents arms so they couldn't lift your foot. having the arms would also explain the double chamber as well. A brute force wrist release? literally pealing the attacker off of you.