1. Yes. NAT only works for information coming from your network, so it's not a factor as far as a Web server or FTP server is concerened. UPnP is an easy way to setup a router, to basicly let everything through. Again, easy to setup, not secure at all.
2. Yes, it will forward the requests, but you have to specify the IP address that it should forward it to, which means your network would best be setup for static IP addresses.
3. Manual port forwarding allows you to forward only ports that are relevant. This allows your server to be more secure. If you know all the ports on your computer, why there open, and how to secure them- you're ok. If not, go with manual port forwarding, just for the sake of security. There is a lot of trojans that scan for open ports, and UPnP doesn't know it's a trojan coming through- it just sends it to the port it requested.
Hope this helps. For more info on NAT, you can go to Howstuffworks.com, or here's the link http://computer.howstuffworks.com/nat.htm