Symbol used to indicate having done a substitution

The problem with your writing in (1) and (2) is that these expressions are not equations, as you claim. Using an equivalence would make sense in the following context:

Consider the equation $$f(x,y) = 0. \tag1$$ Subsituting $9-q$ for $x$ and $6+a$ for $y$, one gets the following equivalent equation: $$f(9-q,6+a) = 0\tag2$$

If you insist to use an equivalence, which I would not recommend in this case, you could write:

Setting $x = 9-q$ and $y=6+a$, one gets the following equivalence $$f(x)=0 \iff f(9-q,6+a)=0$$

but (3) as you write it does not make much sense. And once again, I would simply avoid any equivalence symbol in your case.