How can I attack a Triple-Block cipher with 2 keys (like 3DES) with a cost of ≤ 2⁵⁶
As far as I know your attack is the best attack known, unless something better has very recently been published.

Please note that for DES as the basic cipher the chosen $A$ may not work, but you can choose another $A$ and try again

Also, for a generic cipher with $k$ bit key, the complexity is $$2^{k+1}=2\times 2^k=O(2^k),$$
as $k$ increases.