1. Yes, it's a great starter system. In that price range I would probably look at the Seatool/Fisheye FIX housing instead for reasons highlighted in #4. You seam to be looking for a very flexible camera, though, and I must say that flexibility is not the strongest point of the G10 due to it's lens design.
2. Optical TTL should work with any available housing. (A housing with no window for flash would be, well, dumb!)
3. To start, you can skip macro lenses. The G10 has pretty good macro built in. Stacking more than one would turn a G10 into a microscope, IMO. With 14+ megapixels, you can also crop pretty aggressively. After diving with the system you can decide if you want or need more macro power.
4. The Patima comes with two ports. One allows you to use the entire focal range of the lens. The other allows you to attach macro lenses, but limits wide angle. I prefer the Seatool housng because the standard port allows the full focal range, and the optional one give true wideangle. The downside of the G10 is that no housing allows both macro and wide-angle to be used on the same dive. Ikelite comes closest, but the wide angle is modest. This is what I mean about the G10 not being terribly flexible.
For a more flexible system, I would look at the Sea & Sea DX-G1 or soon to be released DX-G2. These accept the full spectrum of wet-mount lenses right out of the box. There are others that do to, but the aren't as wide or as good at close focus. The S&S cameras aren't perfect (no camera is) but I think they suit what you've described as your primary goal.
Vignetting -- the condition resulting from a port blocking the corners of the photo. It is most noticeable as darkening of the corners, but also causes a softening of focus in the corners. In the worst cases, you literally get a round image surrounded by black.
