If we consider post-mortem rights to matter morally, then something like necrophilia or defiling someone's body after their death would be immoral even if they don't experience it (obviously) and even if they don't have any family or loved ones around to witness it or know that it happened. As an extension of themself, their dead body has intrinsic moral value as far as an obligation to treat it respectfully in accordance with what the person would have wanted or been okay with, not merely instrumental value that it serves to loved ones or the environment. And since we consider a person to have more moral value than the environment (otherwise it could be ethical to kill people to remove their environmental impact), even if it was more harmful to the environment to dispose of a body in a certain way (e.g. standard burial or cremation) over other methods, it would then still be ethical to dispose of it in a way the person either opted for or was likely aware would be done, rather than a less commonly known/practised and more invasive yet eco friendly method such as sky burial (putting their body on a mountain top and letting vultures tear it apart).
In other words someone's bodily autonomy extends into death because they lived in their body their whole life, have a personal attachment to it as part of their identity, and just as they likely wouldn't want it violated while alive (even if they were asleep for example), also likely wouldn't want their body used for something disrespectful or really anything other than a standard form of interment (process of disposing of a body or putting it in a final resting place) that they would probably be aware would happen when they died, or is as generally uninvasive/dignified as possible, unless they specifically consented to something different or made a particular request for what would happen to their body.
IF all of the above is considered true, then (or just in general) wouldn't it be unethical/disrespectful or a rights violation to preserve a human's shrunken head for hundreds of years and then have it in an oddity collector's shop to sell it to people to display in their houses?
Very true, I agree with your points. Just like procuring a file of CSAM, procuring a shrunken head or even something like an elephant's tusk would be imo contributing to a demand for more to be made, as well as perpetuating a culture where those things are desired or even accepted to a degree, which could in turn lead to even more morally unsound methods of producing/acquiring them.
However I would also add that I think even in the hypothetical where accessing or even storing/viewing some CSAM files somehow didn't contribute to any more being produced or shared by anyone, it would still be fundamentally unethical to access it/store it/view it, because while the most clearly abusive component has already happened, continuing to view or use the product of those actions is further violating the child's right to not have themselves commodified or exploited like that, and disrespecting their right to privacy... for the same reason that a peeping tom is violating someone's right by spying on them in their privacy, even if the person doesn't know it happened (except in this case, it's a violation on top of another violation - the child has been exploited, and then people are further violating the child's rights by viewing it).
This aspect of something being fundamentally unethical even if it doesn't contribute to more bad things happening in a measurable/utilitarian sense but in more of a deontological way where the action itself is violating certain moral duties by disrespecting their bodily autonomy, is where I'm coming from by thinking that using/displaying the dismembered body part of a person is unethical regardless of whether doing so contributes to more of that product being created.