This isn't going to do much, but you can use a double length runner as a harness (if it fits your body well). I do that for canyoneering all the time. Use a clove hitch to attach an autoblock biner if you use one (I do, every time, every rappel). Practice first to get comfortable with it, of course. Again, with the weight of the ropes in consideration, another 6 ounces isn't a big deal here, especially for descending 3 technical canyons. I usually bring the runner when I have to do just one or two raps.
This is a non sequitor from the talk of just gear, but I'm a huge fan of the autoblock (mounted below your belay device, never above). But the length of the autoblock cord has to be just right (within an inch really) to balance between safety and usability (I use 47 inches before tying double fisherman knots, for double-rope raps, slightly shorter for single rope).
Are you bringing rappel rings? I didn't see them on there.
I hike off trail in the GC a lot and have never found a compass to be useful. I just bring a topo map as my only navigation (my trusty 1994 trails illustrated, the best version made IMO).
I would probably require a wet suit for this trip (I do *not* do well in cold water). The one time I've done a wet canyon it was in May, in good weather, with a 3 mil wetsuit, and I was freezing (but survived). Similar in elevation to the GC.
Also, I once rapped onto a log floating in a pool and managed to avoid an otherwise inevitable swim. Could you rap onto your packrafts if necessary? ;)