1. Post-script to Mosiah 7. Hem is also a name with a royalist echo to it. It's an Egyptian word that means "person", and means "Majesty" in the expression equivalent to "Your Majesty" (hm.k). So Ammon is described as a nobleman, and Hem and Amaleki have royalesque names. And Hem and Ammon both have Egyptian names -- Ammon is a very old name in Egypt, the name of a god, whose position developed into early virtual monotheism. Don't know about Helem.
2. If Limhi knew Benjamin's last words (presumably because Ammon reported them), then he may have deliberately modeled himself on Benjamin in summoning everyone to the temple.
3. Verse 12 is a sort of verbal pin that ties together a lot of what the Book of Mormon is about through the missions of several men. Limhi wants to read the record of the destroyed Jaredites, and needs a translator; Mormon is composing the record of the destroyed Nephites, knowing that there will be a future audience who will read it in translation; and Joseph Smith translated the verse of the dead Nephite historian talking about the record of the dead Jaredites. And why do we want knowledge of these dead people? To "know the cause of their destruction". Mormon must have seen the Jaredites as an analog of his people.
4. At this point, verses 19-20 come like a punch in the eye. "Doubtless a great mystery is contained within these plates", but men "will not seek wisdom, neither do they suffer that she should rule over them!"