Item #1 seems (possibly) intended to summon from Presbyterian thread participants a parallel response to the thesis of Jeffery D. Johnson's book:
The Fatal Flaw: the fatal flaw of the theology behind infant baptism (Fwd by Tom J. Nettles).
I have a copy of this book, however I cannot find it right this moment. Thus, I have to "borrow" (risking inaccuracy) second-hand a summary of the book's thesis, which I expand and paraphrase as follows:
The New Covenant, with its attendant sign of baptism, is unconditional, whereas the Sinai Covenant (but in practice the whole Old Testament form of religion) is conditional. Ergo, advocates of infant baptism ERR when they allege essential continuity between the form of religion--including internal as well as external aspects--both prior to and after Christ's incarnation. Continuity is vitiated by a fundamental difference in the kind of covenant basis for religious expression.
As with most books on baptismal theology from any side of the debate, the offering has a primary audience closer to its own convictions than any from the contrary side, who it might seek to persuade. It is exceeding difficult to not assume what should and must be proved; and the more effort that has to be plowed into establishing by reasoned argument agreed upon postulates, the longer the book and the more involved the case-to-be-made gets.
Perhaps one way to respond to both that book's thesis and the OP request for a comparative "fatal flaw" in the Baptist covenantal perspective is: to challenge the book's thesis (as I have put it above) with a contrasting claim:
There is no difference in the essential (beneath the visuals) form of true religion, whether the time in view is prior to or after Christ's incarnation. There is but one covenant (of grace) across multiple administrations; the New Covenant is the continuation and (yea more) the fulfillment of an original covenant expression made with Abraham. Ergo, opponents of infant baptism ERR when they overlook or minimize the unity of the covenant of grace; while not distinguishing Abraham from Moses.
I am not pretending to unpack the entire logic of the Presbyterian position contained in that claim, any more than I'm attempting to dismantle the book-length defense of the first thesis. Someday, perhaps, there is a book review (also not a dismantling) in the offing that could appear in the pages of
The Confessional Presbyterian. Each thesis is predicated upon its own set of axioms and arguments that give it stability; and also make it largely impervious to anything other than an internal critique (i.e. challenging the consistency of its premises).
But I think the two theses seen in contrast can serve advocates of both sides, when they compare the claims made. They reveal something of the distinctive priorities belonging to each. It could lead to new combined rejoicing in what both sides actually agree on, even as they come to meet on shared ground from very different starting points.