1) I completely agree with Rewobs that it is a "poorly defined" "liltle used" "no common standards" area in UX.
2) Regarding "copy" specifically, I'm pretty sure the "only somewhat commonly accepted one" is, just as you say, option-drag means "copy, rather than just move".
(*) Note however that we immediately seem to be in disagreement, because on a Mac it is option-drag (to copy), whereas you report it is ctrl-drag (to copy) on Windows (I can't remember; and don't have a W machine at hand) - !
3) I observe that the 3D / game / modelling field is one field that heavily, indeed essentially, uses keyboard modifiers with mouse/dragging operations.
Now there's a critical point on that: famously, every major app is in total disagreement and disentanglement from every other famous app in that field. It's chaos/hell!!!
"Everyone knows" that Unity are "completely f'ing it up" viz a viz Autodesk products (after all - everyone uses Autodesk products, right?) and indeed "everyone knows" that Unity are "completely f'ing it up" viz a viz Unity3D" (after all - everyone users Unity3D right?)
(It really took me a full YEAR to get in to my fingers rotation, etc, with unity's key combos. Infuriating!)
You often hear 3D dudes talking about what a nuisance it is getting finger memory on various products since there is hopelessly no standards or even coincidental alignments.
So, this is worth bearing in mind in terms of your question: in the 3D field (where modified-mouse is used inherently and constantly) there is notoriously NO such standards!