What up, Meneyazwun?
I will answer some of your questions from the narrow standpoint of my own experience.
1. Is Christianity a necessary evil?
For me, I do not believe that it is; but I believe that in some ways it was a useful evil in my case. I grew up in the Christian Science belief system, whose dictates caused me to behave in ways which I consider to be admirable and in which I do not currently behave due to my lack of belief in objective morality.
2. Is it better to destroy Christianity from without or change it from within?
I believe that, if "better" means more conducive to the destruction of Christianity, then changing Christianity from within is better than destroying it from without. I believe that, among the people with whom I have interacted, criticism from outsiders is given less credence than influence from insiders.
My next comment has to do with the following passage of yours:
"[I]t seems to me that openly stating the political agenda of church taxation precludes it being a true political position. "
How? I find this to be a non sequitur. Also, define a "true political position."