Earlier quoted context omitted.
Not perfect synonyms, but we do have shorter words in the same semantic ballpark: virtue, vigor, zeal, concern, care, pains...
I'd say the first three are completely off; "concern" & "care" better (I also heard "grit"), but not quite there.
"Care" I think is the closest monosyllabic equivalent. Diligence and attention to detail are specifically exhorted by "take care", and caution by "use care". Conversely, one may disclaim them all with "I don't care".
It's a short word and probably an old one, so it's heavily freighted with denotations and connotations alike, and finds much use outside this context. But if you want to say "conscientiousness" in fewer than five syllables - which is a sensible thing to want - then I think "care" must be the most accurate word with which to do so.