Even though this may be a hypothetical scenario, I wouldn't be surprised if designers were ask to create patterns and developers asked to implement things similar to what you have described.
Personally I don't think there's such a thing as 'dark patterns' because it is the intent behind the particular design or implementation that makes it unethical. That is, you can implement a particular in the best interest of a user but due to poor execution it might end up causing harm to the user.
But for the sake of answering your question, I think the question is around whether deliberating making information that would affect user decisions harder to find in order to 'nudge' or encourage them into making a particular choice is unethical, and if so how can we minimize the harm.
In a way your question basically serves as the answer because if
The premium option is pre-selected which is double the price of the
Then you can simply remove the pre-selection or make the price different more obvious to the user.
This pre-selection isn't very obvious to the user and the 'continue'
button is already lit up to indicate you're ready to continue down the
I think there's nothing wrong with highlighting a call-to-action that relates to the next step in the process flow/purchase funnel if you address the issue with the pre-selection not being very obvious.
It would be better to understand what the constraints are with your design choices and why you feel like you are being made to do something that would be not be in line with your ethical boundaries. But from your hypothetical question the obvious answer would be to address the exact issues as you have described to make it less harmful to the end-user.