The Crimson Binome
Hero
As the 5E rules are nice enough to point out, this sort of thing is going to vary by DM. Personally, I would narrate those two situations in vastly different ways.If the players don't know how many h.p. the monster has (and they shouldn't!) and the DM is doing a half-decent job of narration, the end result should look exactly the same: each hit is clobbering the monster for between 1/3 and 1/4 of its total hit points and thus should be narrated the same.
The PCs really shouldn't be able to tell whether they're putting 5 points into an 18 h.p. foe or 15 into a 58 h.p. foe.
Of that much, we are in agreement.Confirmation rolls for crits don't negate a success; they merely represent a chance for an additional success.
A '20' is already a success in that you've (almost certainly) hit; now the crit roll gives you a chance to succeed further and really clobber the thing. Failing on the crit roll by no means negates the success already achieved, that of hitting the monster for damage.