Hey Michael, great model thus far (a lot more in depth than I expected from the mention of a lamp.) I am mediocre at best when rigging, but if I'm not mistaken, to solve your upper arm hinge problem, you have to (as NEKOSEI mentioned) create a bone for both peices of the arm. Next, right in the model window and go to New>Property>On/Off.
Under the properties tab (View>Properties) right click (Ctrl-Click on a mac) your new property and pick New>Relationship. This will bring up a new action. First (IMPORTANT) CLICK THE COMPENSATE MODE BUTTON at the top; this looks like a red block ontop of two grey blocks. Now then, right click the first bone you would like to apply this to, and select New Constraint>Orient Like. A medicine dropper cursor will come up (this part is about selecting the bone you want to relate it to,) and you then select the second bone. This should solve that problem.
That's the way I did it, and if there's a better way, someone please correct me. As far as the null: create a new null (New>Null in the model window) and place it at the base. Once again, create a new property by right clicking and selecting New>Property>On/Off. Right click the property in the Properties window, and select New Relationship. Once again an action window will be created. Right click the lower arm bone, and select New Constraint>Aim at. Once again, the medicine dropper will come up: select the null. I believe that should handle it.
Peace,
J Man