Well, for six paths if it is allowed to be used, since there Edos, the ratio rules applies, I think.
As for that list and stuff, well this rule is new, meaning it only applies when it was implemented. I'm assuming Ryuuji isn't coming back, so don't look at that as you not being able to teach (Which you don't assume already)
As for the leadership role, don't assume because one is the teacher, they had a leadership role. Just means they taught the technique. Only real perk is not nobody usually teaches for free, so you might haggle something.
As for the Edo list, well.
Kamui-Hazama. That's one role.
Tsuyo-Trev. Another cause of human path rule.
Shinro. By himself. Don't know who taught him.
Naito, talked to Tsuyo about it, not on the list.
As for why it can't be reversed engineered, well it's not allowed with most techniques. For this to be possible, they would have to have a scroll to copy (Mind you, the scroll contains secret seals, likely hard to just guess at) and know how to use Edo and the kata. Very hard feat to do. I mean, I guess it is possible, but it doesn't really apply to things like hiraishin, Jiongu, etc and people generally don't do it.