There is a separate minimum wage for servers. Last time I saw it (years ago) it was like $2.75USD

Yes, though many states have their own rules http://www.dol.gov/esa/whd/state/tipped.htm

Yes, that is the case. I think it's dumb and wrongheaded, but it is what it is.

Officially, you're supposed to tip 15%. 20% is just easier to calculate.

Waitstaff in any restaurant you've heard of are probably not being paid below the minimum wage. I'm pretty sure the waitress we tipped at L2o was making solid money without my gratuity. It's just a social convention.

I'm still under the impression that standard tip in the US is 15%, but I usually tip 20% if the service is good, if only because it makes calculations is easier. Shoddy service that's not bad enough to notice is 15%; anything less jumps a quantum level down to zero.