My take on that one is that both combine to pull Delapore down into the furthest caverns; it is the confluence of learning about the rats, and also that they symbolize his heredity calling him back down through both the chambers of the Abbey and through the history of his family (and beyond, into the history of the place, and even into prehuman history), pulling him down into those "nethermost caverns", where he is ultimately swallowed by his heredity (and that of the human race -- the beast from which we emerged). That, to me, is one of the things for which Nyarlathotep stands, for example: the "Crawling Chaos" in an earlier piece he wrote (and a title used for this being again in The Dream-Quest of Unknown Kadath) where he was the harbinger of the chaos and decay of the universe itself.
As for the traps... that's an interesting one, and one without (I think) an easy solution. However, my take on it is that this is to indicate that the rats are a very real thing, even if not real "physical" rats... they are the presence of history engulphing him because he has actively sought to revive something dormant -- the traditions and history of his "house" (both literal and figurative). The others, however, do not bear such a close relationship -- their interest is more academic (though Thornton certainly pays a heavy price) and Norrys may be seen to represent the present, that has swept away the De la Poers. In a sense, this makes the entire "presence" of that house vampiric, with a desire to feed on the present -- the living, if you will.
Some of these are just my own take on some of the symbols here; but they are what I make of these aspects....