19 lb including food for 6 days sounds fantastic to me!
"My rain gear sucks, admittedly. But I did survive Washington on the PCT with that set-up. Do you think I'll be just as miserable on the JMT?"
I did the JMT last month, and I believe it's usually drier in August than July. Your rain gear is more than what I brought, and I was fine.
"I can't see needing the Patagonia down sweater unless it's too cold at night for my sleeping bag. I get cold below 40. Should I leave it home?"
I was comfy at night, even at the higher altitudes, with a 15-degree bag, tarp, long johns, and a lightweight wool sweater. Not sure if you need both jackets. Your leggings would presumably be the equivalent of my long john bottoms if you're going to sleep in them.
"How are the bugs anyway? As you can see from my list, I'm a bit paranoid by them. I survived Oregon PCT in July last year and still haven't recovered."
Bugs are likely to be *much* less of a problem in August than July. In July, I was happy during the day with long pants, long-sleeve shirt, and sometimes a head net. At night I was good with the addition of 100% DEET and a second layer of clothing. I slept with a headnet, and that was fine; I'd think you'd be fine with the bug net tent, and you could even consider ditching it and just sleeping with the headnet.
"Any suggestions for something I've forgotten or should maybe leave home?"
moleskin, duct tape?
compass? (maybe not needed since the JMT is so easy to find, but I brought one)
If you don't use trekking poles, maybe think about tarp setup when camping above tree line. I brought a 1-oz collapsible pole for that purpose.
Have fun! Hope your Achilles tendons treat you well!