16-bit files take up twice as much space on your hard drive.
Hardly any photos need 16-bit. The only time you need 16-bit is when you are doing really significant editing in Photoshop. Like, changing areas from really dark to really bright, for example.
Because exporting is a stupid Lightroom thing. The concept of exporting stems from the concept that software controls your files. No. YOU control your files.
The sooner you ditch Lightroom, the better.
That's right. You choose Jpeg for the format, and yes, of course you always keep the colour profile embedded. Then, when you press the "Save" button, another little window will pop up, where you choose the quality level.