They prolly learned skin diving through PADI, so the snorkel stays in.
1) I have heard of vertical currents, though I don't know how strong they get.
2) Yes a freediver will lose buoyancy at depth.
3) The depth varies for every individual. I've never checked my buoyancy in just trunks, so I don't know what my neutral depth is.
4) Short answer is no, I don't think so.
