Thanks for this thread. It was bugging me as well.
In the Coppermind it also mentions that Sigzil is a Fourth ideal Windrunner (from his bond with Vienta). Does that info comes strictly from TSM? or from somewhere else? Is there any way that in WaT he swears his Fourth ideal and we haven't realized?
Crazy idea: what if by breaking his oath in order to save Vienta was his ideal? Kind of "I will do whatever is in my hand to protect someone I love, including breaking my oaths" (quite far fetched, I know, but I'd find it very strange for it to be a mistake since Brandon has written a full book about Sigzil and I guess he knows him very very well)