As long as it doesn't mimic the browser. (Another reason why user agent strings and other ways websites can identify the user's browser are a bad idea).

It doesn't matter if it mimics the browser. If you are trying to stop the browser from protecting the user as intended, it is a dark pattern, regardless of how well you camoflage the attempt.

In principle yes. But why would you imitate the browser if not to mislead the user? And who wants to mislead the user but has qualms imitating the browser?

People do all kinds of mental gymnastics to justify doing stuff they know is wrong. I would be unsuprised that people use the lack of camoflage as an excuse to justify using a dark pattern.