I think you and I have a similar style. I love to have a story ready to tell, and I really want my players and their characters to experience it. The trick is to make it seem like it happened organically and was all their idea in the first place. With my players, I've pretty much accepted that I'm going to prep about 3 times what I need, and then improvise for half the session to cover the insane schemes they come up with, and re-use stuff in different contexts so they still get the cool story bits, but on their own terms.
I tend to run about 80% pre-made adventures, and that can be worse in some cases because the author makes assumptions about what the typical group will do, then I make assumptions for what my players will do, then I learn (over and over again) that they will surprise me every time. In four Zeitgeist adventures so far, the final outcome has not matched the author's expectations four times.
I ran my group through the 4e Scales of War path, all the way from 1 to 30, with a few self-made side adventures sprinkled in. There are entire dungeons that got cast aside in the wake of "I'll just Spider Climb the tower and drop a rope down, and we'll start from the top. Oh, the boss is on the top floor? sweet!" Who am I to say that's an invalid answer?
In another adventure ( [MENTION=1288]Mouseferatu[/MENTION]'s excellent Last Breaths of Ashenport), after we'd left the "investigate creepy happenings" portion of the adventure and moved into the "cultists will kill your face off" part, the group triggered two encounters at once by sneaking around to the evil altar encounter, borking up that fight up mightily, and then trying to flee through the encounter that was supposed to be a warmup. What happened? Two characters died (one dramatically in the bard's arms the turn after he rolled a 1 on his Heal check) and they left that town, never to return, adventure uncompleted. What really happened? I leveraged their characters' fear of that level 8 adventure to make my own level 18 sequel a year later, and they loved it!
What I'm trying to say is let them fail. Let them fail mightily, but let the story continue. I'd probably go with the simple "kobolds capture everyone" setup that everyone has mentioned, but I wouldn't hand-wave it. The dying characters are still dying and need to be stabilized. The 10hp guy is in trouble. None of the kobolds want to die either, so they'll ask for his surrender and then start making crappy healing checks to keep everyone else from dying. Dead PCs are just meat, but live PCs can be ransomed or sacrificed, so they want them alive. If someone dies, they die and can start up a new PC in the cages. It keeps the drama up, even for the players whose characters who can't take actions. Heck, let them make their own kobold-powered healing checks if you want.
The beauty of point buy is you can make an exact duplicate of the character who just died if you don't want to take the time to make a new character mid-session and really liked that build you were looking forward to trying out.