You're lacking a bit of understanding of how NAT functions in some of your questions which is causing you some confusion I think. I'll try and go through one question at a time and forgive me if I miss anything.

1) Remember that switches direct traffic from your computer right to the router and back again, unlike hubs where you would see all the traffic on the network. Essentially with a network switch you're seeing ONLY the traffic flowing back and forth between your computer and the devices you're sending/receiving from. Switches are smart that way, they'll direct traffic right to you.

2) NAT works by utilizing two different networks - one the larger Internet WAN and a local network (your 192 or 172 network) with a router that directs traffic between them. If you were to use nmap with a /24 you would be only scanning everything from 0-255 in the last digit of the IP address. SO for example if your IP is 192.168.1.123 and you type nmap 192.168.0.0/24 you're doing a scan of computers between 192.168.0.0 and 192.168.0.255 - which would be why you would get no results. Try nmap 192.168.1.0/24 and you should see a result from your own PC

3) As I said with NAT essentially you have two IP addresses - one on your own local network (the 192) and one given to your router that operates globally on the internet. Go to Google and check out whatsmyip.org to see what I'm talking about.