No need for me to re-debate or rewrite what I spent a book working through. But, suffice it to say:
I spent a lot of time reading and re-reading the passage you quote. Which is, btw, an account by Tokimune, not Sokaku. And nowhere whatsoever have I ever written that anyone could have questioned Sokichi's courage or valour, least of all Sokaku. I don't remember whose bought my book - and thus, I do not know if you have. but suffice it to say that I spent considerable time proposing a possible reason why a) I believe that Sokichi was the direct teacher of Sokaku in aiki b) why Sokaku didn't say so.
Just a couple of tangent thoughts and questions. We can see from a few sources that Takeda told his students not to teach "aiki" to just anyone. Aside from the nature of aiki being THE secret, could this also have come as a "teaching" from Sokichi? Something that Sokichi "instilled" in his unruly son, just like other lessons? A lot of times, the simple answer is the best one. Aiki being THE secret to his skill is pretty much enough of an answer on why Sokaku stated not to teach it to everyone, but then again, things with Sokaku weren't exactly simple.
I guess if we look at "aiki" as being the skills that Sokichi taught Sokaku, then having Sokichi instill in Sokaku the need to keep them secret and such would seem apt. However, if Sokichi just taught Sokaku a ... rudimentary base, for lack of a better term, and Sokaku being a genius took that and transformed it into something a good bit better, then that changes things. Put simply, was Sokichi as talented in aiki as Sokaku? Or did Sokaku take what he'd learned and used his innate talent to make it into the aiki we know now?
When people later asked Sokaku where he learned this fabulous, martially awesome stuff (called aiki), do you think Sokaku could have picked someone else to attribute it to? Someone like Saigo, who any person with a grain of research would find out the obvious truth that it couldn't have been him (Saigo). If Sokaku had resentments from his father's treatment of him, it would be one way of getting back at his father.