Here's my thoughts...
1- "How geeky do I get? Is it reasonable to talk about R-values of sleeping pads, the water absorption rates of different fibers, and the nuances of LED technology?"
I think it's completely reasonable to be geeky. The average backpacker today is intellectually a touch higher than the average anybody. My opinion, ymmv. But getting somewhat technical is a good thing. Maybe limiting super bight tech talk to shaded sidebars...maybe even have a title for the geeky tech-talk sidebars, like: "geeky tech talk sidebar." Either way, it'd be cool if you included that more detailed discussion without bogging it down, like Jordans "Lightweight Camping & Backpacking." good book, but occasionally got tired of the technical.
2- How can I defend this book against inevitable attempts to dismiss it as a lightweight book?
You may not have to defend it. I wouldn't write defensively at all, either. Andrew, the things you're accomplishing lend a legitimate sense of weight and credibility to the techniques you've attached to. There will always be opponents and proponents to your views, whether it be lightweight vs. Traditional, butter vs. margarine, blonde vs. brunette. Those that question, ask this: what are they doing? Healthwise, where do they stand, now and long-term, with joint issues, etc? You have a platform, your adventures have been successful and impressive. Let that speak for you.
3- Given the changes in the publishing industry, am I correct in taking the self-publishing route, and printing a pre-edition that will be out for about 3 months before the full edition is ready?
I'd give a chapter one "teaser" and on that teaser to keep an eye out for that book, as well as your AYE book. Website address, etc. Teasers have caught me before, hard.
4- What are the major weaknesses of other lightweight books? How can I avoid them.
I think arguing for your view defensively is something to avoid. Being matter of fact backed by your experiences is key. You have a couple hikes under your belt, I'd listen to you if you said something. Wouldnt necessarily go off and take it for gospel, but would check it out myself, all the while noting your experience level it would carry weight for sure. Also, don't get too technical in the main text, but have illuminating side bars where you dive deeper.
Good luck, looking forward to your tomes.