If one has purchased the game over steam but uses a pirated offline key for V20, does that count as theft?
Yep. You're using un-authorised stuff still, despite paying for it.
You're contributing to the issue.
Trying to think of a metaphor, but they're all kind of silly...
Like if I work at a recycling plant, but don't recycle. It's okay, because I work at a recycling plant and that makes up for it.
(it doesn't)