It's not just that 17 years is "more than 'less than a year'"...it is quite sufficient time for a master swordsman who is reasonably good at teaching to turn the goods over to a bright student. Particularly if that was 17 months of daily training sessions, and the teacher and student had a very good rapport, which are two things i have read allusions of.
The rule rather than the exception in the life of the koryu in modern times is: the headmaster dies, and some number of senior students fail to find adequate motivation to follow the new headmaster. Since every ryu has its own unique criteria and process for licensing, you wind up with some talented individuals without menkyo kaiden going outside of the umbrella of the ryu. And if they happen to attract lots of students and build organizations, the reaction of the folks back at headquarters is going to range from dismissal to petulant derision. Which can then be magnified by foreign students who grew up in cultures where people speak their minds more plainly than they do in Japan.
The unequivocal testimony of senior practitioners of KSR is that:
a) Inaba was given access to a very limited portion of the curriculum, for a very limited period of time, and that the combination of circumstances was insufficient to give him "the goods" (which goods are swordsmanship as practiced in KSR, as distinct from a marked improvement in his swordsmanship above that of run-of-the-mill aikiken).
b) part of the original terms of license, if you will, were restrictions on the circumstances under which he could teach and the way in which he could represent the material.
c) a number of his students, some of them very senior in the world of aikido have -- for many years, with his apparent blessing and/or collusion, have -- widely advertised what he taught them, and by extension what they are teaching, as KSR. In so doing, they have embarrassed themselves, called their own integrity and good faith into question, and tarred everyone else doing aikido by association.
d) in a counterpoint to your final graph, one might suggest that (increasingly) the rule rather than the exception in aikido is that, faced with the absence of one or another key element in the art of aikido as taught in their line, a number of students of one or another shihan seek out talented individuals who have received more substantive training in that area from a koryu art, but no longer abide by the school's restrictions on when, where, and to whom that material may be taught. They then study with that individual and publicly represent their studies with that individual as legitimate koryu. Sadly, because aikidoka almost all refrain from speaking their minds in a forthright fashion when it comes to the obvious foibles of senior aikidoka, that this scenario raises questions regarding both the technical accuracy of the instruction, the lineal accuracy of the representation, or the integrity of the individuals involved in passing off as genuine koryu arts what are, at best, unauthorized knock-offs and, at worst, cheap counterfeits, is not only studiously ignored by all concerned, but rationalizations for this bad behavior are actively promulgated.
In this light, the decision by the KSR head office to decline to instruct anyone actively involved with aikido seems not at all "cultish." Rather, it makes a great deal of sense, even if one does not credit their explanation of the essential contradictions between the arts, on the grounds that they simply wish to insure that they are only taking on people of good character -- by their definition of "good character," in which definition understanding their ground rules and abiding by them is a key element.