Those who claim that the "modern Protestant creeds" (like the Westminster Confession) are consistent with an inerrant Biblical canon have a central problem that few of them recognize.
In essence, what they are saying is that the earliest Christians who articulated the creeds (the Apostles Creed, the Nicean Creed, the Chalcedonian Creed, the Athanasian Creed, etc.) screwed up, so others tried again - and screwed up again, so others tried again and finally got it right. The most obvious problem is asserting that those who were closest to the actual events and early prophets somehow didn’t understand them and their teachings as well as those who came later. Using that logic, and that logic alone, Mormons must understand better than orthodox Protestants, since Mormons came later. That’s not the logic I would use, but it is exactly the logic many use to justify their adherence to the specific creeds they accept and their rejection of the earlier creeds.
These people claim the later creeds are more accurate; Mormons claim the earlier creeds are more accurate. The most ironic part of their argument is that Protestantism was founded on the idea that the Catholic Church had misconstrued the earlier creeds - that Protestantism was necessary to return to a more correct version of what the Gospel had meant prior to the changes over time - to “go back”, if you will, to the earlier, “correct” understanding. The problem is that this leaves people arguing both sides of this coin ("The later creeds are more accurate, because they better reflect the earlier understanding, but any creeds developed even later are inaccurate - because they came later.")
My suggestion for these people is simple: "Pick one stance and argue it; just make sure the one you pick doesn’t justify the Mormon position even more than your own."