> *CAVEAT LECTOR*
> I _KNOW_ I'm doing an *ATYPICAL* install.
> I *REQUIRE* Grub to be its own partition.
> The Debian 8.4 installer at least recognizes that some users
> would desire Grub to be in its own partition.
Not sure if this is helpful, but could you redo the install in expert-mode, during which you get to pick and choose among the installation steps. Then, at the step for installing grub, you could do that in a shell or, perhaps expert mode gives more access to the commands being executed?