3. Whatever you meter on in spot will cause the camera to suggest exposure setting that turn that spot to middle grey. Find something that IS middle grey in level and meter on that and lock exposure.
4, and 5, Hunting is not unusual in really low light which is why I suggested using an AF mode that offers the assist light. AF-S, center point will do that. But the camera assist light is bright white and not patterned grid projection of a SB600-910 speed light. Mount your SB600 and go to a dark room, Put the camera in AF-S mode and enable the assist light. Turn off the speed light and try to focus on a wall. Notice the bright white AF assist like come on. It will greatly help AF in low light. Now turn on the Speed light which will switch the AF assist lighting task to the flash. Press the shutter release 1/2 down to start the AF function. You will see a red grid pattern is projected that is dim and hardly noticed by subjects but is very important to the AF system. That pattern lets the AF sensor detect phase differences better and really improves AF speed and accuracy. Using the speed light without emitting flash can be done by 1/2 pressing the release and locking focus as soon as it locks on(AF-S mode) but remember, the DOF is really shallow so any movement of the subjects in the focal plane will impact focus. With the AF locked, turn off the flash and shoot away on any subjects on that focal plane. All this is much too fast to manually focus, if the dot turns on, a 1/th second later the subject might be out of the Focal plane unless you have stopped down to give a deeper DOF....which further lowers signal to noise ratio. The dancers and anyone else do not like direct flash and they are mostly used to someone blashting away with a point and shoot close to them and assume the worst while a flash comes out. If properly used, they would not mind at all because they would hardly notice it. First off, you need to point the flash away from them, so the light is bounced from the room itself and is highly diffused. The apparent intensity lowers by a factor of 1,000 or more. A straight on directed flash tube in a concentrating reflector is really really intense but we do not need the whole scene at those levels we just need to raise the effective ambient a few stops. The point and shoot flashes they see in their face and dislike are slow, they last a long time compared to a SB600 pulse. It is so fast that the light detection and transmission of the information the brain and the brain registering it never completes fully. 1/10,000 of a second is pretty darn fast. That is why dogs and babies do not react at all to diffused flash. The pulse of light is gone before the brain fully registers it. A point and shoot produces a different more upsetting reaction however due to its slower rise and fall time of the strobe tube. If you tried using flash in a way that helped you and was not noticed by the cast, the images would look good, they would look good and they would not even notice. Bad use of flash gives us all a bad rep. 6, I do not understand the question or even the set up used, can you post an image of it? 7, If you were manually focusing and had the lens opened up, the front line was definitely not in optimum focus. DOF is pretty shallow, and even if the dancers looked like they were at a constant distance from you, they weren't. AF is going to be a lot more accurate and faster for moving objects. That is why I was suggesting the AF assist lighting. That is still in Single Servo so it is not going to track like AF-C but AF-C without the assist light is just going to hunt. the Assist only works in AF-s mode.
10, I can't think of a better bang for the buck and least distruption of marital harmony than a 85 1.8G for all round use besides the dim light dance shooting.
Single shots will likely produce more keepers than burst because either manual or AF-S is not going to track and they ARE moving so any shots away from the first acquired focus, will be out of focus. Do you have a friend who will loan his D3s to you? ;>) That would solve a lot of these issues. Shoot at f/4-5.6 for more deeper field and just let the magic of a usuable 51k ISO do its thing. A D800 can use 25k for small prints or for a newsletter/web posting and deal with this light. Stan St Petersburg Russia