I have no dog in the fight, and wish "enormity" had never developed a normative undertone, but I strongly disagree that said usage is anything close to archaic.
If you Google "enormity," the dictionary definitions it displays before the results are:
1.the great or extreme scale, seriousness, or extent of something perceived as bad or morally wrong.
"a thorough search disclosed the full enormity of the crime"
2.
a grave crime or sin.
"the enormities of the regime"
Merriam-Webster claims this is not the exclusive usage, and that enormity can mean "immensity" without normative implications when the size is unexpected. But the very example it cites, from Steinbeck, involves the "enormity" of a situation in which a fire was started.
That said, I agree that "enormousness" is an awkward word, which I do not use. I'm left to ponder the enormity of my own pedantry.
As a native English speaker, I can't say I've found this to be the case. "Enormity" does tend to be used for dramatic effect, most often on moral issues, but I don't think that makes Scott wrong to use it here.
I don't know if I've seen "enormousness" before this thread.
Since enormous is from Latin, stems tend to be Latin. `ness` generally only is morphologically productive with Germanic roots, kindness, happiness, etc.
When I visited Iceland, I remember a sign in English that said a cliff was insafe [sic]. `in` being a Latin morpheme, and safe being Germanic.
Enormous : enormousness :: huge : hugeness.
Enormity = Immense scale of evil (e.g., the "enormity of the holocaust")