I haven't been to Philmont and don't know the weather for when you are going, so these suggestions may not be worth much.
(1) Instead of taking a 2d boxer brief AND a 2d pair of shorts (you already have the convertible pants), think about the lightest pair of running shorts with a liner that you can find. This will be enough to wear for swimming, hiking or while washing and can be worn under the pants too.
(2) Driducks pants seem to be very fragile (according to others here) so keep that in mind. Weather will dictate if you think you need a rain shell for your legs, but you could take an extra garbage bag for a low-tech MYOG rain kilt. Mike C. made a rain kilt out of a pair of Driducks pants once, but I don't have a link to that thread now.
(3) You shouldn't need the sock liners, especially if you are taking hiking shoes instead of boots. If you do take a 3rd set of socks, take 3 sets of light wool socks (it's amazing how several ounces can be saved by weighing socks and picking the shortest and lightest you have/can buy). If you take a 3rd, save them as sleep socks until the last day.
(4) The Cap 3 sleep layers seem a little warm for that part of the country, at least in summer, especially for the legs. However, I don't have the Cap 3 weights in mind, so switching to some lighter merino tights or REI silk tights may not save that many ounces.
Otherwise, it looks good. When are you going to Philmont?