even just a few mV can be lethal in the right spot.
That's an awfully small spot (inside the body). Say a few mA instead so you can drastically widen your target area. You're far more likely to hurt yourself with a low voltage source if it has high current capability (i.e. car battery) by the side effects - the heat and molten metal spraying over you from shorted contacts, etc.
that's like saying you can drown in just an ounce of water, but you're far more likely to spill it and then slip, fall, and crack your head.
To some extent that's true, but the first situation is far more likely to happen. Car batteries are dangerous - there's a reason rescue crews cut the supply. It's quite easy to cause a small explosion by dropping a tool across the terminals. Other high current low voltage supplies are similarly dangerous. Try playing with an arc welder. (No, don't do that if you don't know what you're doing!)
Depending on who you listen to it seems like 15 to 30mA through the heart can be lethal.
As the body resistance is a few kilo ohms you need well above 12V to create that current.
Am I correct in thinking that the resistance of dry skin is a few kohms, and inside the body (particularily the heart & brain) could be much lower?
Yes. Anytime you have chemistry with ions - like salts in the water of your body - you have lower resistances. That's why I agreed with your statement about a few mV being potentially life threatening - if you rip open your body, get the electrodes right on your heart or brain, and hit the right spot. The electrodes will need to be close together. I think it's an open question if your more likely to die from the mV potential, or the insertion of the electrodes into the center of a vital organ. Your body exists in a sea of charge - anywhere clothes are touching your body, you routinely generate hundreds of mV potential through rubbing. kV levels are easily possible, if you try. You really don't need to worry about low voltage potentials - please remove that 9V battery from your tongue, though.
That's why it's more useful to talk about current - you can assume the required potential and sufficient charge is there, whatever kV levels are necessary. As deBug pointed out, small currents are potentially lethal, as they can cause heart arrhythmias - irregular hearbeats that your body can't recover from. Ironically, higher currents are actually safer - your heart stops completely and can then recover normally. That's what they do when they apply a defibrillator and zap you. The higher currents are safer up to the point that they really start to fry you.......
this thread should be split...
Sure. But I'm not sure what else there is to say....