It seems to me that there were other avenues - that the method of learning in the strict soke-sensei-student, Shu-Ha-Ri may be more a modern way to pass on "traditional" arts?
I note that researchers have written how time-in-tutelage grew considerably in modern times, that previously it was much less time months or a few years, before licenses were given, even menkyo kaiden.
Also, I'd have to check Friday but was in Kamiizumi that gave out a certificate to a peer after a very short time of study? This makes sense - fellow experienced warriors sharing knowledge, picking things up from each other, or perhaps one as the elder brother and the other the younger, and the teaching of the elder more a "finishing school" or rounding out of a total education. Very different from the sensei-student relationship portrayed today.
The story of Yagyu Munenori(? or another....) giving the certificate to the guy with NO training based on his assessment of the man's attainment in the mental aspect of the "Way"...
Or Musashi's having "no teacher?" While clearly not exactly true, certainly in his writings much more of a focus on personal training,insight, and understanding rather than devotion to the words of a teacher....