Until Grudem (and Morey, and Van Till) rejects flagrant circular presuppositionalism engaged in by their opponents, on the ground that it’s flagrantly circular presuppositionalism. 
They know perfectly well that circular presuppositionalism is worthless for being circular, when other people do it.
Insofar as they stick with abductive presuppositionalism, there’s some real value in testing the hypothesis of their presuppositions by checking how well the data fits. (Morey, for example, often does this–or something close enough to count in the short range–once he gets past the opening chapters of his book.) But they have a tendency to treat the presuppositional set as not being a hypothesis for abductive testing. They would much prefer it to be treated as a necessarily granted presuppositional set; but then when the question comes up for why we should necessarily grant the presupposition they want to show why by appeal to the data, as though the set was being abductively proposed.
That’s cheating. And they know it’s cheating, which is why they go out of their way to reject it when their opponents do it (or when they think they can paint their opponents as doing it. Either way fits the principle.) Otherwise they would have to admit that an opponent could do that with the exact same parity of result, leaving their own position (arrived at by the same circular strategy) at no advantage.
Moreover, you yourself know better than to engage in circular presuppositionalism. This is why you don’t propose to argue from the necessarily granted truth of specifically Calv theology (and, btw, every Protestant Armininan I have ever met also considers themselves Reformed non-Catholic ) that specifically Calv theology is true. You propose to argue from “things like the resurrection and the Trinity and you affirm that salvation comes from Christ alone”, i.e. things we already hold in common as Christian believers (or trinitarian Christian believers anyway). You consider specifically Calvinistic theology to follow as a chain of (logical) consequences from that shared position, and you want us to see it also.
Whereas, I consider the resurrection, salvation from Christ alone, and universalism to follow as a chain of (logical) consequences from trinitarian theism (as well as being testified to exegetically in the scriptures). But I’m not setting up Trinitarian Universalist Christianity (of this-or-that variety) as a necessarily presumed truth set and then arguing that universalism (much less trinitarian Christianity) is true. Similarly, you (at least seem to) know better than to think we would (or even should?) be at all impressed if you set up Trinitarian Calvinistic Christianity (of this-or-that variety) as a necessarily presumed truth set and then argued from those presumptions that Calvinistic soteriology is true.
Your reply to Alex, in other words, doesn’t appeal through circular presuppositionalism (flagrant or otherwise). It does proceed by locating common ground shared by both sides and using that common ground as a presumption relative to any following argument; but that is not the same thing as circular presuppositionalism.
Off to Amazon to shop the used vendors…)