Just a thought or two from a beginner...
I do not have any background in Japanese and I've only been practicing aikido for a few months. Perhaps my ideas have some small validity though, since perception counts for a lot.
Anyway, I recently found a website that listed the Hombu Dojo requirements for being affiliated with aikikai. I don't have the link at hand but I probably found it through a URL posted on aikiweb. The site also described the requirements for recognition as shihan, shidoin, and fukushidoin (sp ?). If I remember correctly, fukushidoin status is for 2nd and 3rd dan, shidoin status for 4th and 5th dan, and shihan for 6th dan and higher.
Even before I started practicing, I assumed that senior students had a responsibility to help junior students. The ability and willingness to help also affected their rank, promotion, etc. I see this during my own practice with even 3rd kyu students frequently helping me with a technique. Carrying this a step further, I see the yudansha in the dojo helping the 3rd and 2nd kyu students as well as helping out as guest instructors during some practice sessions. Finally, I see the chief instructors leading the advanced classes.
It may be that my perception of the terms has moved outside of their linguistic roots but I see measuring improvement in technical skills as very difficult in the higher yudansha ranks. Teaching, on the other hand, is more easily recognized and can be evaluated. Certainly length of time leading seminars, quality and quantity of publications, and overall respect are plausible means of comparison.
Summing up my somewhat rambling post, I have always assumed that the definition of shihan included teacher. Even if it started from master - apprentice, perhaps it has come closer to Master Teacher.