Yeah, this is one of those solutions that, had I been writing, I would have ruled as not actually workable. Takes too long, Voldemort or a Death Eater sees the threads and breaks them on general principles, nanotubes don't actually have enough tensile strength to reasonably slice up everybody at once consistently, and so forth. I pretty much filed any tactical violence plan under "not practical".

Still, not my story. It's not out of keeping with the rest of the stuff in HPMOR.

High odds that Voldemort escaped. He's been extraordinarily hammy this whole time, and he called all the Death Eaters together, killed or crippled a few, and then explained his evil scheme to the hero before giving him a countdown to his inevitable death, while leaving him armed. It seems very plausible that Tom Riddle was tired of Lord Voldemort, and decided to retire him via dramatic massacre. Why make his new body a snaky freak-show, after all?

On the pedantic nitpicker trivia side of things, you don't have a tank of "oxyacetelene". An oxy-acetylene rig uses a tank of oxygen, and a tank of acetylene. But Harry is probably not a welder, and neither are the Weasely twins, so nobody involved was equipped to notice the problem.