Your transition modeling challenges very likely originate with trying to model the camera using a higher level of “smoothness” than it actually has.
Your camera was probably originally designed with straight lines and simple circular arcs, flat surfaces, edges which were cylindrical, and corners which were spherical. Simple arcs and straight lines are excellent fits to the curved corners and straight edges in your photographs of the camera.
G1 is the highest level of continuity geometrically possible between a straight line and an arc, a flat surface and a cylindrical surface, and a cylindrical surface and a spherical surface. This is basic geometry, not a Rhino imposed restriction.
Forcing a higher level on continuity such as G2 between such curves and surfaces generally results in regions with higher curvature near the transitions. In order to maintain the same radius over most of the “corner” and have G2 continuity between the corner and the flat surfaces the arc/cylinder/sphere must be moved in and the transition zone extended into the straight line/flat surface area. Again, this is due to basic geometry. This type of surface refinement is non-trivial, and not a good exercise for learning the basics of “smooth” surfaces.
To effectively model your camera use a combination of straight lines, arcs, flat surfaces, cylindrical surfaces and spherical surfaces.
To learn about creating high quality models of “smooth” surfaces start with an object with such surfaces.
Descriptions of continuity levels: http://docs.mcneel.com/rhino/5/help/en-us/popup_moreinformation/continuity_descriptions.htm