The second one isn't a sentence. It's two dependent clauses. 1. When you're dead, it won't matter who dug your grave. 2. A lot of things that could have been said haven't been, and some tropical areas haven't been mentioned at all.

The first sentence, "Little it will signify, and we to be making clay, who was it dug a hole through the nettles or lifted down the sods over our heads," suggests that the speaker believes that what they are doing (making clay) is insignificant in the grand scheme of things, and that it doesn't matter who originally prepared the area for the clay-making by digging a hole or removing the sod. The sentence could be interpreted as a reflection on the fleeting nature of human activities. The second sentence, "How much remains unsaid, and what large tropics have received not even passing mention," implies that there is a vast amount of information that has not been communicated or acknowledged. The phrase "large tropics" might refer to broad, overarching themes or ideas that have not been fully explored or discussed. The sentence could be interpreted as a statement about the limitations of human knowledge and communication.