Determining grammar decidability
I don't really understand your reduction, but there is a very easy reduction from
$ALL_{CFG}=\{G| G \text{ is a CFG and } L(G)=\Sigma^*\}$.
Given $G$, Simply output $(G,G_{\Sigma^*})$, where $G_{\Sigma^*}$ is a (concrete) grammar that generates every word.

And the problem $ALL_{CFG}$ can be shown to be undecidable with a very nice (but elaborate) reduction from the word problem for TMs.