Hence "really", to indicate one has to parse and contemplate the upshot of the rules. What do they amount to?
I mean, when the text
explicitly says ability checks are for resolving success, and even the later section you point to explicitly says, "You determine the consequences of...ability checks,[...] When an ability check equals or exceeds the DC, the check succeeds." It's pretty clear that
ability checks themselves are about success or failure. DM judgment
in response to a check, on the other hand, may involve questions of consequences.
Notice the "may," or "can," as used in the text. Emphasis added: "When a character fails a roll by only 1 or 2,
you can allow the character to succeed at the cost of complication..." "You
can also use this technique when a characters succeeds on a roll by hitting the DC exactly..." "However, you
can choose to take such an exceptional roll [nat 20 or nat 1] into account when adjudicating the outcome." Note also the (repeated) use of referring to the check itself as "succeeding" or "failing." It still seems pretty clear, even in the DMG 242 text, that checks are about success vs failure of a given task. It is only what happens
after the check--that is, the description and adjudication of what "you succeeded at the task"
means--which
can involve DM intervention as to consequences, though it need not
have to.
The rules themselves are (somewhat surprisingly for 5e) quite clear about what they're for. This is an area where the "DM-curation," to use your term, crops up
after the rules are triggered and resolved, not in the process of triggering the rules.
I don't believe ordering implies optionality. The text can't be legibly piled on top of itself: some must come later.
That said, I can see that if you decide ordering implies optionality you are likely to draw different conclusions.
I mean, yeah? It's literally the very last portion of the Using Ability Scores section. If it were meant to be the heart and soul of...y'know,
using ability scores, you'd think it would get somewhat higher billing!
Even if it weren't, the text makes it pretty clear how optional this is. It repeatedly uses "you can choose..." or similar phrases ("consider adding...", "when you introduce..." meaning you aren't always introducing such things, referring to them as "flourishes and approaches" rather than as the core function, etc.) Flourishes are optional: "a decoration or embellishment," "showiness in the doing of something," etc. "Approaches" likewise implies that these are some possible directions, but far from the only ones; one could, in fact, decide that it's unnecessary "to make things a little less black-and-white," that black-and-white resolution is perfectly adequate.