Yes, yes, and yes. The reasoning behind these etiquette rules is that it is rude for a relative to beg friends to do for another relative what she should be doing herself (assist the relative in transitioning to a new state in life); even on occasions on which gifts are traditionally given, it is rude to specify exactly what should be given; and it is rude for the guest of honor to be in any way involved in deciding what she should be given. All of these actions, however common, are abuses of hospitality and the generosity of others. As an authority on these kind of matters, I recommend Miss Manners.
Sin requires knowledge and consent. It is possible that some people think it is perfectly fine, based on current practice, for relatives to use the major milestones of life as an opportunity to pass the hat for each other. They are objectively wrong about that, but they do not sin if they are not acting maliciously.
It’s not necessary to shun people to make your feelings about this practice clear. If you would otherwise enjoy going and genuinely wish the person well, then go, bringing a gift that you think appropriate and that is not “registered for” or placed on a “shopping list” distributed by the host or the guest of honor.