Hi Wildboars
1.DC didn't come with a mission editor but you can manually edit all the missions using a text editor such as wordpad.
A little more tricky to use is a program called Koralle which I believe is still downloadable from subsim's download section.
Even better is kriegstanz which does not allow you to edit existing missions but lets you create new ones which can then be easily edited.
http://www.brookview.karoo.net/Kriegstanz/index.html
Highly recommended.
2 DC defaults to "no external camera" - a real pain.
Go to REALISM settings and enable it.
3. Sorry. Kriegsmarine was never finished. It doesn't have a campaign written for it.