Emuna is a Nesayon all the time, even if you are a Navi. Even Avraham Avinu had a Nesayon. Although Hashem spoke to him directly, it was still a Nesayon.
Kalev went to Davenat the Kvarim of the Avos and Imahos. He clearly knew what was right and what was wrong. Nevertheless, attitudes rub off. During a person’s lower moments he forgets his better moments. It is easier to bulldoze than to build. Even though a Navi’s Nevua is more clear to him than actual eye sight, he can still reflect at a lower moment down the line that perhaps he was imagining it.
The fact is that there is no Krias Yam Suf today. Coming to a mature conclusion, being Modeh El Ha’emes, and sticking to it is not easy and is not guaranteed. While it is important to address questions that arise there is no need to explore more questions unless you are doing so with the goal of addressing them, when you know that you are equipped to do so.
Da Ma Shetashuv does not mean to engage the Apikorus in conversation. In fact, Halomed Davar Echad Me’hamegush is Chayav Misa.