Good job on a vast topic. A few thoughts:
1. Breaking a long day into two may sound easier, but it's not. Schlepping overnight gear on some the excellent routes you mention makes them way harder, and less safe. Narrows, Buckskin/Paria, etc are incredibly fun - but not if you twist your knee, fall and sprain your wrist, or get your sleeping bag wet. Heaps, Imlay, Kolob, become struggles if carrying camping gear. Mobility and nimbleness is key.
2. The gear you do carry however, cannot be ultralight. It will break. Jared Campbell and I tested the upcoming Fastpacks from Ultimate Direction in Zion NP this March, and I immediately changed the spec on the bottom panel to made it stronger. Sacrificing a little extra weight for durability is very wise.
3. By far the easiest ways to be an "Ultralight Backpacker" while canyoneering is to: 1) Don't backpack - do it in a day; 2) Don't bring much gear (because you don't need it and what you do bring has to be strong).
4. For example, for our Zironman route (across Zion NP off-trail) my clothing selection was: 1) White, long-sleeved, tech-T (white fabric is cooler than dark skin); 2) 2.5 oz wind shell; 3) Running shorts; 4) Floppy sun hat; 5) Fullbody 4mm wetsuit with hood, booties, gloves. Everything in between is a waste - no fleece, no down, no layers, forget all that - if you're in the sun you're too hot, if you're in the water you're too cold. There isn't much in-between to worry about. I also don't bother trying to keep anything dry.
Have fun, Buzz