Perhaps Iomedae could consider investigating whether matters are changing in Cheliax in a way that will, on net, damn a lot more people to Hell, rather than presuming the results She will find and looking only in places where She expects ill news.
Part of the reason why Cayden Cailean is trusting Nethys is that Nethys has not simply asked Him to take things on faith, but built an agreement that is in part conditioned on watching how things play out. A foolish waste of time if Cayden Cailean knew Nethys as well as Nethys knew Cayden Cailean, but that, of course, is impossible with Nethys.
It was Cayden Cailean's estimate, at the end of all Nethys showed Him, that there was, indeed, no better way to let this play out than to let Iomedae expend some of Her energy on proving to Herself - and anyone else watching - how certain things play out.
And if Iomedae - or certain other gods - were the sort to immediately believe Nethys about what was shown, why then, Cayden Cailean really wouldn't be able to believe Nethys, given Nethys's incentives. If Nethys's claims had the power to immediately persuade any god without further demonstrations of facts, and not just to partially persuade Cayden Cailean, well then, maybe Nethys would just say whatever was to His own advantage, after all.