So are we assuming that the ancient Kurosawa (the first generation of sealmasters) did things entirely on their own, instead of leeching of of other sealmasters who weren't Kurosawa?
Err...No, that's not what I was saying. Let me say the same thing with more words and see if it helps.
What I meant wast that the first Kurosawa reached a practice of doing
X that took advantage of their ridiculous bloodline. (For example they copied everyone else's sealing style...but what the practice was is not really relevant.)
They passed this practice on to the next sealmaster as the tried and true, a.k.a. best method to abuse Kurosawa bloodline for sealcraft. The new sealmasters took that as granted, since sealing research is dangerous anyway, and never really tried to push the envelope and see if they could figure out something else. Or maybe a few did but they reached some other solutions than modularity. This keeps going on until this day.
Enter Hazou, a Kurosawa that leaves the village but manages to find a sealcraft teacher anyway. He doesn't know what is the "right" way to use the bloodline in sealcraft and has no preconceptions of sealing imposed on him by the clan. He was also born and raised outside the clan, meaning he only shared in the clan culture through his mother, so the indirect influence from the clan that the previous sealmasters received is also diminished.
Hazou is an irreverent munchkin and his cognition can be described as atypical even for a Kurosawa (poor guy, he has a SV thread running through his subconscious). He knows there must be a way to use his bloodline to cheat in sealing so he keeps learning stuff from Kagome as he tries to figure out his own style.
Maybe Hazou gets inspired by this weird secret Mori technique that Keiko calls "Logic". If that doesn't make sense, maybe they just had a little workshop about the optimal ways to map out a conversation and Hazou stumbled on the idea of having these idealized "gate" thingies. You can put stuff in them and the gate uses rules to put out something different. He could even use simple symbols to differentiate different sorts of gates so you would know at a glance what happens in that part of the discussion flowchart, maybe he could even make small pieces of paper with the right symbols and then you could switch them around in the fly...
...and that's how he ends up with the idea of Modular Sealing.