True. However, it's only one assumption. Whereas proving that the universe is fathomable takes many, many more. So many I don't think any religion really offers a full explanation for things. There are always mysteries. An unfathomable universe simply takes that idea to an all encompassing extreme.B) Assuming the Universe is unfathomable is itself an assumption.
Well, as you said, there's really no way to prove or disprove many of these hypotheses. Occam's Razor is just a generalized way of thinking about the soundness of a hypothesis.C) Counting the number of assumptions on a topic like this probably isn't the best way to determine the plausibility of a theory.
It's not meant to offer an explanation. Why must you have an explanation?D) It's still a bit of a cop-out that doesn't really offer anything in the way of explanation.