Upload & Sell: Off
1 I'm sold on the idea of purchasing a manual lens vs an autofocus lens. I'm less clear on which type of extension tubes to purchase. It seems that non-automated tubes are adequate for macro so long as your lens has a manual aperture ring. Would most agree with this claim?
I rarely use anything else. You need to ensure that for conventional mounting the adapter operates the aperture when you mount the lens i.e. it engages the close-down button, lever, or whatever. If you are going to use the lens reversed you ideally need a lens in which the detached lens works a manual diaphrgm e.g. Nikon.
2 I'm confused about what exactly is needed for TTL metering. In the above scenario (non-automated tubes), would an SB700 attached to my Nikon d600 properly meter? Would I need to first set the camera into a different mode (stop down metering mode) ?
I'll leave that to someone with specialist knowledge. However, if the only linkage between camera and system lens is mechanical to the diaphragm there should be no problem, even with the lens reversed.
3 When discussing a manual-focus lens with max aperture f/4, Ken Rockwell says that f/4 is fine as long as you don't mind a darker view finder. Am I correct in interpreting this as, "the photos will be great, but you won't be able to see or frame your shots as well" ?
f4 is quite a wide aperture. I rarely use (stopped down) anything wider than f8, more often f11 or f16. The electronic viewfinder can give a much brighter image than an SLR. With practice, you can work well with dimmer viewfinder images.
4 What is the best way to offset a mounted flash so it is not directly above the lens? Will a long, trapezoidal diffuser without flash offset be an acceptable compromise?
I use an L bracket.
5 Rockwell says that while ~100mm lenses give you a decent working distance, the perspective at which they take photos is unnatural. Any differing opinions on this? How much does unnatural perspective matter when taking pictures of really small objects?
Because of the shallow DOF this is probably less of an issue with macro than with other fields of photography.
6 Recommendations on manual focus lenses for Nikon macro (to be used with extension tubes)?
If you have a Nikon 50mm you may like to start with that reversed.
See what others use. I use quite a few lenses, of various makes and focal lengths. I also use high quality supplementaries in for of some lenses.