I don't really understand what you mean in your whole post, but the extra assumption ##A(\lambda,\vec a)=-B(\lambda,\vec a)## is only necessary in the original Bell inequality. In general, when you have 3 random variables ##X##, ##Y##, ##Z## with values in ##\{-1,1\}##, they always satisfy the inequality
$$XY + XZ - YZ\leq 1\text{,}$$
which can be proved by checking all possible combinations. You then get an inequality between the correlations:
$$\left<XY\right>+\left<XZ\right>-\left<YZ\right>\leq 1$$
So far, this is only a purely mathematical inequality. In order to apply it to Bell tests, you need to plug in the variables that are measured in such a test (##A(\vec a)##, ##A(\vec b)##, ##A(\vec c)##, ##B(\vec a)##, ##B(\vec b)##, ##B(\vec c)##). Non-contextuality and locality ensure that you have only 6 variables, instead of possibly infinitely many. However, for the original Bell inequality, this is not enough. You need 3 variables instead of 6 in order to plug them in the inequality above. Hence, you need an additional assumption, which is ##A(\vec a)=-B(\vec a)## (for all ##\vec a##). Now you get:
$$\left<A(\vec a)A(\vec b)\right>+\left<A(\vec a)A(\vec c)\right>-\left<A(\vec b)A(\vec c)\right>\leq 1$$
Now you can use our third assumption and replace every second ##A## by a ##-B##:
$$-\left<A(\vec a)B(\vec b)\right>-\left<A(\vec a)B(\vec c)\right>+\left<A(\vec b)B(\vec c)\right>\leq 1$$
With some algebraic manipulation, you can rearrange this into the original Bell inequality:
$$\left|\left<A(\vec a)B(\vec b)\right>-\left<A(\vec b)B(\vec c)\right>\right|\leq 1+\left<A(\vec a)B(\vec c)\right>$$
If you use an inequality with 4 variables (the CHSH inequality), you can drop the ##A=-B## assumption. Nevertheless, you still need non-contextuality and locality in order to break down the number of variables to 4.