Delta airlines booking mistake?

I just booked my flight to Jamaica with Delta, who I'm not used to. I'm used to flying with Southwest but couldn't this time. Well I purchased my ticket and didn't see the option to choose seats so I paid for my ticket and just expected to get my assignment during check in. But my friend who I'm going with was able to choose hers.
So now I'm screwed because we're more than likely not going to sit together. Also I may be stuck with really bad seats since I couldn't choose. Is there a way for me to fix this? Any Delta flyers out there that can help me?

Why does a green box always show up on my private part when I'm scanned at the airport?

Literally whenever I go to the airport and I go through the full body scanner I look at the screen that shows the outline of a body and a green box always shows up in my hoo-ha region. At first I thought it could've been the buttons on my skirt but regardless of buttons it always shows up. No one else I see go through has that. Is there a reason? It's getting really annoying lol

Question about connecting flights?

This is probably a dumb question, but I've never flown before. I live where there isn't a really major airport, so when I look at flights it's from my city to ATL (connecting flight). Then it's a layover to the city I want to travel to, and it says ATL to Destination City. My question is, when I arrive at the city I'm traveling to, does the flight say from ATL to the Destination City, or from My City to the Destination City?