[Mihai]

The black stripe is normal I think, it's a reflection of darker parts of the bottle. You could model the bottle thinner, but that wouldn't look realistic.

You can use the default lights as a starting point and ofcourse you can change their intensity in the material editor. I usually use lumens as a measure of how much light they should emit. The skylight will be pretty bright compared to the default lights so it's normal the scene becomes much dimmer if you turn it off.

It depends also what camera settings you have.