While the death rattles do show glimpses of the future, I am fairly confident that they do not exclusively show glimpses of the future, just primarily show glimpses of the future (as that’s more relevant to the story, the ones we’ve seen will be mostly future).
It makes far more sense, magic wise, for the rattles to be glimpses of the spiritual realm, and thus could be future, past, or far distant present. All time and space is blended together in the spiritual realm; I don’t see why Moelach’s Connection to the dead would only show the future - that doesn’t seem to make sense with the magic? Rather, I think what they see are ‘truths’, as the Shin sailor so aptly named them.
I could, of course, be 100% wrong.