>I always treated Aperture Mode as a means of controling DoF, >whereas Shutter Mode was for controling camera & subject >blur (or lack thereof). If that was the starting point would I >not be better off in S mode and set the shutter speed to say >1/200 and let the camera adjust the aperture and ISO to suit?
Sure, you could use Shutter (S) priority mode, though depending on how low the ambient light level is, you might run out of ISO before getting a correct exposure without decreasing the shutter speed as well. You could also use a shorter focal length which will increase the aperture (decrease the f/#) though using wide angles is not flattering to the human form. 30mm - 35mm should be ok, I don't have an 18-55mm handy to check the aperture at 30-35mm.
Personally I would use Manual exposure mode.
>Q3. What is the highest ISO I can use and still get reasonable >4x6 prints? (These particular shots will never be severely >cropped or enlarged.)
I don't have a D3200 though it should provide reasonably good IQ at about ISO 3200 and possible up to ISO 6400. I am extrapolating the numbers using dxoMark.com sensor ratings comparing the D3200's high ISO performance to the bodies I have. I would recommend taking a few test shots at various high ISO settings and see for yourself since each person has their own sense of how much noise is acceptable. If the noise is objectionable, you can use Noise Ninja, DxO, or other Noise Reduction Software in PP.
>(I am thinking I could make the necessary settings in P mode >and have them there when needed.)
Give it a try. Experiment and learn what works best for your particular requirements and style. Program mode will not keep settings in a memory per say. In P, S, and A exposure modes: if the ambient conditions are beyond of the range of settings available, the camera will indicate HI or LO in the display. Good Luck and Enjoy your Nikons!