So, it's been a couple of months since the morning my computer caught fire. After all that drama I owed you guys/gals an explanation of how things played out. It's overdue, my apologies for not posting this sooner.
Short version: while the drive manufacturer did not admit fault on their part, they quite graciously replaced my hardware, and the store I bought the drive from covered my (minimal) other damage and cleanup costs. I was very happy with the way both companies handled the situation.
Longer version: a couple of days after the fire, I got a call from a Global Support Manager at HGST, who was at least as shocked by the incident as anybody else. We eventually agreed that I'd ship the remains of the computer out to California for analysis. The Microcenter store where I'd bought the drive (every single person I dealt with at that store was wonderful, you should shop there if you're ever in Marietta, Georgia) facilitated the shipping for me.
We had to ship the whole box because the drive was completely encased in melted plastic. HGST finally carved the mechanism out of the case and did their analysis. HGST did not admit any fault on the part of their drive, but they were also very careful to not point any fingers in any other directions.
After they'd sent me the analysis slides and we went through a couple of rounds of questions, HGST offered to replace the computer via the Microcenter store, and I was fine with that. Like I told their representative, if I'd wanted to sue my way to a "lottery ticket judgement," I never would have called them in the first place. I was happy to be made whole here and leave it at that. The Microcenter manager followed up by offering to pay the cleanup costs (which amounted to fixing two vacuum cleaners that had been wrecked cleaning up the fire extinguisher dust) and their techs even built up the box for me (which I'd have been happy to do myself).
Best of all, nobody involved ever asked me to sign an NDA, or so much as mentioned the subject. That would have been a deal-killer for me, and I was very pleased by the fact that it never even came up.
The new box works fine, and while the experience was one of the bigger scares I've ever had (as incredibly unlikely as a recurrence would be, I still make sure to power down every morning before I go to work now), I'm impressed with how both HGST and Microcenter stepped up their customer service in a very unusual and not-a-little-stressful situation. Both companies deserve a "thank you, and well done." I'm intentionally not identifying by name the two main people who helped me, but Mr. HGST and Ms. Microcenter, if you'd like for me to do so, please let me know, I'll be happy to make an edit.
Thanks to all of you as well, for all the kind words while I was freaking out in the immediate aftermath.