Decidability of equivalence of two context free grammars
Let me prove that the problem of determining whether $x = 0$ is undecidable.

Let $A$ be a Turing machine that accepts only $x$, and let $B$ be a Turing machine that accepts only $0$. Then $x = 0$ iff $L(A) = L(B)$, so according to your argument, determining whether $x = 0$ should be undecidable.

I hope that this example helps you understand why your reasoning doesn't work. Instead, try to to use the fact that CFG universality (given a grammar $G$ over $\Sigma$, does $L(G)$ equal $\Sigma^*$?) is undecidable.