Fair point about the different witnesses, as it's one of the reasons there are so many variations of the same myth. But I still disagree that Dumac has any relation to Malacath. Firstly because we're told he doesn't, which is simple enough. Second because Trinimac being dead doesn't stop his name being used to infer a reenactment to which Malacath has no mythical ties, and for which he clearly isn't suited as an understudy.
First off, Trinimac is about as dead as Jyg was before SI, which is to say not at all.
Malacath does have mystic ties ton the incident, by proxy of association.
Trinimac was present there, and thus has a connection (obviusly).
Malacath is Trinimac's other, obviusly related significantly to Trinimac. Thus, Malcath is associated with the incident by proxy.