There are some good points you raise, and I think you're really testing the nuance of what nudging is. But nudges are perhaps more basic than you're thinking.

There are some good points you raise, and I think you're really testing the nuance of what nudging is. But nudges are perhaps more basic than you're thinking. Nudges don't need to steer the user to a specific choice, just a behaviour change. Sticking with a conversion flow counts as a behaviour change. Nudges don't need to be simple or understandable. They can be a set of complex changes where causation isn't clear. They just need to get results. The only really hard requirement that would rule out a nudge is if you forced a choice or used financial incentives. If you read the Nudge book you'll see that it's a political book, really. The authors introduce nudges as an alternative to hard regulation. Instead they propose that governments consider influencing behaviour in a softer way, but still leave the escape hatch open for people with strong preferences to choose what they want. This strikes a balance between state involvement and principles of liberty. (Or at least that's their argument.) Because of this framing a nudge is defined mostly by what it isn't. It's not a nudge if it forces a user to a choice; a nudge is anything you do that changes what users do without forcing them. This is what you've done with your series of changes that resulted in increased conversion. You've left all the choices open still, so users have as much freedom as before, but you've managed to predictably change user behaviour in a way that aligns with your goals. In other words, you've nudged them.