The practical side of diffraction on the D800 is because of the large number of pixels, diffraction becomes an issue at about one stop wider than the D700/D3 - and that's about the same as the D300. Diffraction starts to be measurable at around f/8, but is rarely a problem until you get to f/11 or higher.
Diffraction impacts the sharpest area of your image - not the soft areas. If lens quality, DOF, motion, or anything else softens your image, the impact of diffraction is outweighed by the softness (in normal aperture ranges). Diffraction is most relevant at or near the focus plane.
The tradeoff on diffraction is increasing DOF. DOF should drive your composition with diffraction issues being a secondary consideration. With macro in particular, you often are struggling for adequate DOF and stopping down to diffraction impacted apertures. But composition needs would justify stopping down and accepting some diffraction impact. With landscapes, you may find that DOF tables show a small impact of moving from f/11 to f/16, and the diffraction impact may not be acceptable. With wide lenses even f/8 may provide plenty of DOF.
I don't worry about diffraction at all below f/8. I willingly use f/8 to f/11 as needed knowing the diffraction tradeoff is minor. I try to only use f/13 to f/22 only if needed with composition driving that decision. And I very rarely go beyond f/22 for any reason - even with macro.
As for your original question about resizing, the high resolution images of the D800 can be sharpened with a higher intensity and small radius before downsizing to recover edges impacted by diffraction. Then after resizing you can do a second pass of more typical sharpening as needed. This is probably a matter of taste for individual images.