Thanks Mary for your comment and Sean for your valued criticism. If I may respond to them.
The reason I asked about the negative space on the first picture is that I actually feel like you do about it. On the other hand, my mother, who has worked at magazines and ad agencies for over 30 years had a totally different opinion. She has huge experience with stock photos and said not to change a thing, as it leaves a great space for text. Of course, that's a whole other purpose and images for stock photo should be considered with a different eye. For the second image, an almost square crop is probably better.
As for the dark background and the flash look. First of all, I'm not sure I really mind a dark background, as long as it doesn't get too dark (as in the second picture). Macro is all about compromises. I've used a regular flash with and without softboxes, reflectors, etc. and I could never get the look I really wanted. I don't like the shadows a single flash produces, as evidenced by your first example. A softbox helps a lot though. I finally broke down and bought the MT24-EX macro flash, and I find it works great.
Dark backgrounds are difficult to avoid, unless you have strong sunlight, or a close background. The following image was taken with a 580, diffusor and a reflector. The background was around 30 meters again, but thanks to strong sunlight it comes out quite bright.
Contrast this image with the second one I posted, which was in heavy shade, but the background was only 30-50 cm away. Diffusion was not used, and probably would have helped a bit.
Currently I am looking for some way to be able to use the 580 as a slave to the MT24 to fill in the background. It would require some sort of stand/tripod that is light and quick to set up.
Regarding the aperture and the diffraction limit. I am very aware of this problem and rarely go beyond f/14. It has happened, but only when I desperately need the DOF, like you said. What I don't understand is your statement about not going beyond f/10 unless the subject fills the frame. Care to explain that?
BTW, the shots I've shown here are also uncropped.