1) If a manufacturer tells you they take KiCad files, then they will take them. Just be sure the information is up to date.
2) Each manufacturer has their own requirements, try to find them in their website.
3) I believe you mean the board layout editor (pcbnew) here. I recommend using the GAL canvas - use the View menu to switch to the OpenGL canvas, if it's not active already. You can use an interactive router. It tries to find a good route for you. At each point where you are you can click and it will come the next fixed point which isn't moved automatically. Just play around a little and you will find out how it works.
4) If you mean the board edges on Egde.Cuts layer - no, each line segment is separate.
5) Do you mean the schematics editor? There you can start a new line where you left off, no problem. I don't see a need to prevent it, it's normal behavior and often what we want.
6) What do you mean by "box"? A filled polygon (not a group of line segments)?