To the first question, it is an Antitank gun.
In a pinch the army one from Saipan would suffice. Keep in mind that when the Japanese navy and army are in the same scenario, they probably should not cooperate.
To your second question, you are technically correct. The marines would work to bypass caves, or get above them at the front, when possible. Something to keep in mind though is that these are cave complexes, not just a hole in the ground. I personally suspect that the 360° fire chosen as a game simplification. I can certainly imaging a couple of reasons for that.