I think it was clear that Tokimune was to succeed Takeda.
I think at that times Tokimune was too young. Maybe Sokaku wanted to insure his art sucession, just in case something bad happened to Tokimune.
Sagawa was asked to in case of Tokimune's death and there was some sort of written document Sagawa asked for.
But that was when Tokimune went to serve in the military, isn't it?
And, what about the claims from Kakuyoshi Yamamoto camp about him being appointed sucessor by sokaku himself in his latter days?