Some really good points so far. Regarding the mandatory meetings that Charles suggested--don't worry about scaring people away. Seriously, if adults won't even make the effort to come to a pre-trip meeting, they're not going to have much involvement or effect in the troop. The adults who do show up you can probably count on as adult leaders. I also think that by making the meeting mandatory, you kind of generate a team feeling, locker room pep talk-like. Make the trips a special group effort, something they can take pride in having or developing the knowledge to do.
To second Evan's comment, I wouldn't broach the lightening up subject as ultralight, or backpacking light, or whatever. IMO, I don't think I'd even use the packing smart phrasing (some implication of packing stupid). I regularly get customers in the store, complaining of bad backs or knees, saying they can't go backpacking any more. I don't talk about going ultralight. I just tell them that I can easily get them a fifteen pound baseweight--with all the comforts they're used to, and with strong safety allowances. They look stunned, say wow. I say yeah, the stuff they're doing with equipment these days is really amazing. And you can get by with fewer things than you used to be able to, but still stay just as warm and dry.
One thing that struck me is what sounded like constantly different adults on trips. Sounds like they're not enjoying the experience enough to keep going? I've said it elsewhere, but not everyone likes tarps. A lot of people, and virtually all newbies, are more comfortable in a conventional tent. Mountain Hardwear has the Lightpaths right now, $160 and $190, roughly 4-5 pounds. No, it's not as light as a tarp. But it might make some people more comfortable and keep them coming on the trips.
Regarding younger scouts, and not relying on them to teach... Rely on them! Raise your expectations of them, and they'll raise their expectations of themselves. When I was a kid, my friends and I got our Eagles around 14 or 15. We wouldn't have done that so early if we hadn't been constantly encouraged to lead ourselves and teach others. In my experience, young Scouts invariably enjoy and respond well to increased responsibilities.