First off, there is no one technique that works on every hill for every rider. Every hill is different and may require a different technique. For example your climb of 4200 ft in 18 miles isn't very steep so you may sit and spin more. That same climb in 7 miles may require a lot more standing. That being said lets address your points.
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1) Upright position vs chest close to handlebars:
Upright position is better since aerodynamics are negligible during climbing. However, bringing your chest closer to the bars with arms bent allows more leverage by allowing you to pull up on the bars.
First, you should always ride with your arms slightly bent. If you have wide enough bars this shouldn't be an issue (this is an often overlooked issue with larger riders). I don't see how getting your chest lower will really help you get better leverage to pull on the bars. Generally, on climbs you are better off getting more oxygen in your lungs than trying to get a little more leverage to pull up.
2) Shoulders dropped vs shoulders open:
Dropped shoulders allows for a more relaxed position, thus conserving energy. Shoulders open allows for easier breathing.
I usually ride with my shoulders dropped (as you say). It's a more natural position and holding your shoulders back would result in too much fatigue, especially over a long climb.
3) Ankling vs no ankling:
Ankling allows for different muscles to be used, thus allowing big muscles to rest. However, it appears that ankling is actually very inefficient since you're forcing small muscles to take over the bigger ones, thus you'll actually get more tired.
I do this for a stretch or recover on longer rides but it's not something you should be doing all the time.
4) Sitting on the saddle:
Sitting more behind on the saddle allows for bigger muscles to be used. Sitting forward produces more power? I'm finding that there's too much pressure on my sit bones when I sit more behind, even with my chamois...
I don't think sitting forward produces more power, I think it's actually the other way, at least for larger guys. Whoever wrote that was probably a 150lb climber. They have different body mechanics than us larger guys.
5) Straight back vs relaxed back
Straight back allows more core muscles to aid in climbing. A relaxed back saves more energy.
Same as #2 and #3 above.
...I'm curious what you guys do to succeed in climbing long distances. Which of the above do you do, what cassette do you use, how much water do you bring, etc. Is there a more efficient climbing form for big guys compared to other riders?
When I'm doing long (20-40mile 1%-4%) climbs I do a lot of sitting and will often climb in my aerobars because I'm climbing at speed. I'll throw in periods of standing to stretch a bit. My gearing is probably irrelevant to you unless you race ultras but I usually run a 33/44/60 triple with a 11-15 cassette. If I'm doing something with steeper climbs in the 15%-20%+ range I'll run a 11-28 cassette. I also have a wide range of chainrings that I'll use depending on what I'm doing. Use the right tool for the job!!!
The most efficient climbing form is the one that works for you. We all have different mechanics. The best way for you to get up a hill is to experiment and try different things and be flexible. You very well may find that what works for you when you are in shape doesn't work so well when you are out of shape!