One possible explanation is that the horcrux doesn't require a murder to create, but it does require a human brain to restore the backup to. This doesn't seem terribly likely, but I think it would be a elegant solution to why horcruxes need murder.
The prophecy (at least canon - I remember MOR having a slightly different one, but cannot find it offhand) could point to two identities of Tom Riddle. The hero and the villain. Neither can (truly) live while the other survives.
I think the time travel hint was a bit too strong. I basically had two possibilities: H&C is a time traveller with all the world breaking implications, or Eliezer is meta-screwing with us. There's no other high probability reason for H&C to say that right before he obliviated Hermione. If the latter, all other bets are off - I can't seriously approach predicting a work like that. So I'm very glad Eliezer let us know.
Probably cheerfully ignore them, considering magic's general relationship with physics.
That would make for a pretty nasty situation. I had considered throwing a large rock that was transfigured to be smaller, then dropping the transfiguration in the air. This would be even nastier (though maybe not as effective): transfigure a large rock into a needle and throw it at them, then when it's inside them reverse the transfiguration.
I imagine healing charms would be at least somewhat effective. If shield charms don't work this would also be a great dueling technique.
Good point: you should treat your own transfigurations like toxin - particularly if you are transfiguring from a toxic material or to a non solid. If the armor was imbedded in you and turned back to a non-toxic solid (wood, stone) that wouldn't be much worse than steel. I guess you might as well transfigure it out of ice if possible. In this example I think that not transfiguring the armor because you are afraid of transfiguration (a reasonable fear to instill in children, but not adults), and therefore taking a blasting curse to your unarmored chest is worse.
Yep, it's not a great solution when you have any vaguely adjacent friendlies. I was mainly referring to his dream of floating above azkaban and incinerating it to the bedrock. Also it seems pretty relevant considering all the talk about nuclear bombs. They aren't dangerous at all in comparison.
A simple counter example (hopefully shorter and more clear than the other more in depth criticism by michael sullivan) is the scenario where warren had exactly equal priors for organized fifth column, unorganized fifth column, and no fifth column.
p(organized) = .33
p(unorganized) = .33
p(none) = .33
If he was practically certain that an organized fifth column would wait to make a large attack, and a unorganized fifth column would make small attacks then seeing no small attacks his new probabilities would approximately be:
p(organized) = .5
p(none) = .5
So he would be correct in his statement of concern (assuming an organized fifth column would be very bad), even though the probability of no fifth column was also increased.