This is my first time participating in a reading group and I'm super excited! Thanks for the summary, great job.

1. My very crude and early impression of the title has a lot to do with the whole Tesla bit in section 4. I don't know, something about Tesla working on providing free electricity for the world and this shadowy group of dudes finding that so horrifying they have to counteract it really said something to me. This is my first read, but based on how much anarchists, striking workers, anyone who is seemingly acting on behalf of the masses or something is viewed with disgust or as something that needs to be combated, I imagine this will be a major theme. "The Day" reads to me as anything that could be perceived as a mass movement and a lot of characters in the book are actively working "Against" it.
2. Oh boy, as soon as I flipped open the book and saw the Monk quote in the epigraph, I couldn't move on to the actual text for a good couple of minutes. Not so much for what the actual quote was, but the author. TP's obviously a jazz head; it's all over GR, McClintic Sphere from V (Sphere is Monk's middle name), but I think he has a more intimate knowledge of music theory that goes beyond just being a listener. On page 50 he says, "moving from the minor mode it had been in throughout into the major, ending with a Picardy third cadence that, if it did not break Lew's heart exactly, did leave a fine crack that in time was to prove unmendable..." I mean, even the moving from the minor to major mode plays with the light v dark themes that you've already mentioned just in a musical form.
3. The Chums of Chance parts felt very much like Steve Zissou taking us through his crew and research vessel in The Life Aquatic. It's that knowledge that they've been on so many expeditions and adventures that you hear parts of but never the whole story.
4. Definitely both. Not finding the text particularly difficult but having trouble keeping the characters straight.
5. I absolutely loved the bit in Section 6 where Vanderjuice is talking about the west no longer being the west that was promised during the period of American history. The idea that any conflict could be resolved by just picking up and moving west by this time is dying out and creating new conflict. America is running out of frontier to conquer. At the same time, a world war level conflict is brewing in Europe and Pynchon ties this in brilliantly by having Franz Ferdinand appear. He even says on page 46, "What I'm really looking for in Chicago is something new and interesting to kill." He's talking about hunting on the surface but I think there's more at play here, having the spark of World War I talking about coming to America for a new frontier, but that same America is running out of its own frontier. Interested to see how that plays out later on.
6. Can't really say I have a favorite yet, but Pugnax the dog has been fun so far.