Not as much as you might think. Create a MenuStrip control that has the common items on it, and then copy and paste the control to other forms. You can then build on this menu structure, saving you a lot of time. Be aware though, that when you copy and paste a control, the corresponding code does not get copied.
Yes and no. There is no built-in functionality to do this, so you would have to write LOTS of complicated code to allow users to customize your toolbars. A better approach is to purchase a third-party component that provides this functionality. Personally, I think buying a component that supports this is a much better option than attempting to write the code yourself.