Your idea that the death rattles are forward-looking and that on of the death rattles refers to the black castle don't mesh. Unless death rattles can look into visions, they must be able to show the past. Being able to show the past during select circumstances wouldn't even be that surprising, as future vision works by viewing the spiritual realm, and viewing the past should be equally doable because it would work through the same mechanisms. While it is possible that Moelach normally shows the future, it seems like some of the rattles transfer more general knowledge, instead of specific visions, with knowledge of events in the past being part of this.
Edit: Just looked through the arcanum, and it seems nobody actually asked about this. Maybe something for the next signing?