Of course it is very dangerous to touch 340 VDC and don't try it, yes. But 120 VAC is still much more dangerous.
You can touch a bug zapper, though, as its current is very low.
Also, I have touched 170 VDC. It doesn't hurt very much.
Yes, electricity is very dangerous.
Since the electricity is coming half the time, thinner wires can be used which means that its not really a large increase.
I'm not sure about that alternator vs dynamo thing, but it is very stupid - instead of generating DC directly, the alternator generates AC and bridge-rectifies the outputs. I've been wondering why that's the case.