Avram in MD – all good questions, and I’m not going to claim that I know the answers. There is a Gemorah, which learns the concept of money forcibly taken away as the RBSO counting it towards Tzedaka from a Pasuk (not a S’varah or a “why” type consideration). If we really believe this with all of its practical implications (and I’m not saying that we do), then yes, you would have lower (or no) taxes if you gave enough Tzedaka, weren’t an Am HaAretz, etc.
Assuming that having your money taken by force is what makes it count as tzedakah (and I don’t think that applies to U.S. taxes), why would non-Jewish tax dollars going towards a credit for a Jewish family with kids yield any more zchus to the taxpayer than, say, the tax dollars going towards a new warship?
That was my earlier question above. If giving money counts as Tzedaka does being on the receiving side mean you are accepting Tzedaka, or perhaps not. I could hear both ways (i.e. it is reasonable to say that the money is given and done with, and the choice of the middle party to give to specific individuals is their choice and not “giving” Tzedaka to be “accepting” Tzedaka).
So while giving towards government programs Mi’daas or having taxes taken by force of law would be counted as Tzedaka, receiving government programs very well might not be.