The blue dotted line is just to show the return route to the homeport, just to help differentiate between the directions, the red solid line goes out and to all the ports - the tiny arrows also show the way, but they are not always clear, especially on fjord cruises.
As for the cabins, I can't really comment, usually take what we're given, travelling steerage! I sometimes browse the site, but haven't actually used the site to book, but I agree that it isn't great for looking up certain cabins.