I'm not sure this should really count as an "unresolved plot point" seeing as A. it doesn't really affect the plot point and B. The Giant has made it pretty clear this is not going to revealed; it's a riddle deliberately made to have no answer. (Contrast the case of the MitD, which is a riddle with a really hard answer.)
Why is Fyron's son not in SoD?
Isn't the obvious explanation for this one that Fyron's son was not, in fact, killed by Xykon, and Roy simply got it wrong for some reason?