I often shoot the same subjects and conditions that you describe, and I’m not at all sure I would choose a 400mm fixed length telephoto. I often find myself surprisingly close to my subjects, so that framing can be a significant issue that favors a zoom lens. Because a tripod cannot always be deployed, lens speed can be critical even if the lens is IS/VR, (more is always better). The 400mm DO IS has a seriously long minimum focus distance of 11.5 feet, which can be too great for small animals. Since you have already rejected the 100–400 mm IS, you may wish to spend some time practicing with the 300mm f2.8 plus 1.4 TC. This combo has a weight disadvantage of 1.78 pounds, but has the ability to focus at 8.2 feet. As I'm sure you know, it is also devastatingly sharp even with the 1.4 TC, and in my estimation, very well balanced and steady for hand-holding. Oftentimes when photographing small animals, we must get down to ground level, and this can provide an extremely stable shooting position making the 300 f2.8 quite manageable. I have never heard of anyone rejecting a C or N 300mm f2.8 for lack of sharpness and contrast, NEVER. For my $5300 US, 1 in 3 odds of getting a good performer is simply unacceptable, as is additional postprocessing to correct for lens deficiencies.
If this type of subject comprises the bulk of your work, you should consider all available options including the Nikon 200–400 f4 VR and the D2x. This lens weighs 7.2 pounds, focuses down to 6.5 feet, and stands up well to the prime lenses in terms of sharpness, contrast and boke. With Nikon, you also have access to the 200 f2 VR which can yield a very potent 280mm f2.8, 340mm f3.3, or 400 f4 when paired with the 1.4, 1.7 or 2.0 TCs. This setup would weigh a bit more than the 400mm DO IS, be shorter in length, focus down to 6.2 feet, and offer much greater flexibility.
One final comment. The attached images were all taken with an 80-200 f2.8 hand held, at less than 200mm focal length. Without zoom capability, I would not have gotten these images.