Moshe Rabbeinu merited to speak to Hashem with utmost clarity, but he still had doubts about emunah and Bitachon. If Moshe had these questions, of course we can too.
Miriam convinced Amram to remarry Yocheved after originally divorcing her. When Amram decided that was the correct thing to do, it was a real psak he made due to the circumstances. But Miriam made him look at the situation differently, thus overturning the psak.
When one davens, there’s a certain power that one has with even a slightest amount of energy to have major implications. Of course, the more the energy, the more the effect.
Strictly speaking, we don’t fast erev shabbos. The one exception is Asara B’Teves. Why? Because sometimes you need to make sure the situation doesn’t get worse.