もっとも……洞窟の中にいたとしても、冷静に考えりゃ無事にはすまないんだが、それでも外よりはマシだっただろう。
もっとも……洞窟の中にいたとしても would be "Even if you are in the cave..." and それでも外よりはマシだっただろう be "It would be better than being outside".
But I don't understand this part very well: 冷静に考えりゃ無事にはすまないんだが. I know that 冷静に考えりゃ is "thinking calmly", but 無事にはすまない confuses me. I think "すまない" must be another word, because "sorry" doesn't make sense, but I don't recognize it without kanji.