I agree with most of your reply. Except for this:
It's a major breach of social etiquette to decline an invitation from the Captain while embarked aboard his vessel.
First, it just ain't true. You are perfectly free to decline any invitation from the Captain you wish, including an invitation to eat with him. He is not entitled to command performances.
Second, to argue that not attending the Captain's cocktail party is a faux-pas is absolutely laughable. Nobody's checking you off a list, and gobs of people--including me--don't go. It's a silly mob scene that many don't attend.
That said, I completely agree that there's a certain lack of logic to booking a line that stresses the "old fashioned" liner traditions more than most, then try to find ways around the situation. There are so many lines that are more appropriate for people who feel this way, as you point out.