1: By default the router is handing out DHCP address normally this is fine, in your case you don't want the ip of the machine your trying to get to to change so you pick one and set it in the network settings for that machine, again normally you would decide to hand out a range of ip's say 192.168.1.10-192.168.1.50 now any machine to connect will get one of those , then you would set the static one to somthing like 1952.168.1.51. that way the router won't try to give out 51 and your covered.
2: No it's not necessary
3: http is web based traffis like the page your looking at it's actually a port number, imagine a house full of windows 8080 is a downstairs window ,3389 is an upstairs window.
4:The router has a config page for DNS just poke around and set it to dyndns now when your outside ip on the router changes it will notify dyndns and that will change so
www.myweb.com always goes to your router.
The dynamic dns for the outside world is for when you get a new ip on your router from your ISP, if that were to change you wouldn't know what it was, dyndns will translate the change to a dns name
www.myweb.com the static assingment inside on your pc is so that the router will always know to send rdp(3389) requests to that pc if your router gave the pc another ip the settings oyu have set for the rdp requests would be kno good as it would try to send them to let say 192.168.1.51 when your router may have given the pc an ip of 192.168.22.
