Points 3 and 4 also seem to contravene with how you are using the notion of warrant. The notions of "design plan" and "proper function" doesn't allow a non-Christian to appeal to simple warranted belief since that results in appealing to the very thing that is a defeater.
Will, here's the deal. Plantinga has laid out the conditions for warrant---what is conspicuously absent from them (notice) is that the subject be conscious of operating under the proper-function model. All that there has to be is a de facto design plan.