With the Narindar are they fated to be successful unless some interference by the No-God?
Not necessarily. Narindari are cultic assassins. As per the Glossary, some of them (particular Ajokli's) may be granted The Unerring Grace, but it's not absolute and can come and go. Yatwer's Narindari, White Luck Warriors, possess absolute Unerring Grace to the degree that you describe above. Only be rewriting reality (and the gods themselves) can a White Luck Warrior fail.
After thinking about Kellhus and the Narindar it seems that in order for him to survive both of the assassinations, he has to ultimately lose in his attempt to prevent the No-God. To prevent the No-God after the fact would cause a paradox in that he would not have had Kelmomas to prevent either assassination attempt from being successful.
Good observation, and I agree. Kellhus indicates as much when he states that at some point the Consult MUST succeed and the No-God MUST be reborn. However...
Would Kellhus potentially had the knowledge that in order to survive he needed to have some form of contact with the No-God at both junctions, and that the No-God had to be revived? Or is this attributing a greater level of foresight than Kellhus would have possessed. It would seem to explain why he would go back to Mommen to rescue Kelmomas, and why he let him live as he would need him to become the No-God in order to save his past self. Also it would add further reason to why he has him in such a prominent position when Sorweel makes his attempt, possibly he was aware of the risk at that moment.
I don't think so. Kellhus was fooled by Kelmomas. He says as much, but more importantly, his actions bear that out. He doesn't waste the Strength on cracked bowls. If he knew that Kelmomas was as crazy and dangerous as he was, he would have acted on that, as he did with Inrilatus. In short, Kellhus was saved from the White Luck Warriors through luck, not skill or foresight.
I can't quite reconcile it with how he met his end, except the fact he would know he has to lose at that point and Kelmomas has to be used for the No-God. It also relies on him having knowledge of Kelmomas role as the No-God prior to the first assassination attempt in order for him to go back to Mommen in the first place.
I don't think there's any textual evidence supporting Kellhus knowing that. He wanted to kill Kelmomas and almost certainly would have if Esmenet had gone along, but not because he knew or suspected Kelmomas was the No-God. Simply because he was a dangerous wild card.