"If ... Rev. Warren is not entitled to any tax deduction at all, because such a deduction would violate the First Amendment (separation of church and state),"
The First Amendment states:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;
If the tax deduction violates the First Amendment "establishment" clause, then any tax at all on a religion is unconstitutional because it violates the First Amendment "free exercise thereof" clause.
I say bring it on. Then the churches would not be so "scared" of losing their tax deductions (prohibiting the free exercise thereof) if the pastor's of those churches lace their sermons with political speech, such as whom and who their congregation should vote for.