Hats off to languagehat once again, for kicking off another fascinating discussion on linguistics (by way of food).
And American pronunciation norms do work hard to avoid a profusion of schwas, unlike AusBrit norms. British in particular takes collapse into schwa as a mark of refinement and high culture.
However, Restniks do not typically say a schwa in Marcos and the like, and do not do that lengthening you speak of. This might give us pause. We use the “short” o of our on, got, and often, or of everyone’s for (which in fact M-W Collegiate equates with the vowel of law, in its reading of American pronunciation). The question that exercises me concerns this difference between Americans’ and Everyone Else’s practice with foreign words. Is it because our “short” o (quite near to many foreign vowels written as o) is more salient to us and somehow more available, while the nearest equivalent in American speech (the vowel of for and law) is somehow less available to them, and their more available “short” o is disqualified as confusible with the way foreign a is sounded? Perhaps. Strictly, though, length itself seems less relevant. So is stress, per se, less relevant.
And then there’s couscous, still unaccounted for.
(Noetica gets into her/his stride in the comments.)