Just some more context on why the Import dialog box is not the ideal place to do image-by-image work.
One reason is that because nothing is imported yet, nothing is recorded until you click Import. This makes it easy (at least for me) to lose unsaved changes by clicking on the wrong thing and either accidentally closing the Import dialog box (where you have to start over from zero), or select images I didn't intend to select. Compounding this, there is no Undo in the Import dialog box. So if you are going to edit anything in the Import dialog box, you need to play close attention and not screw up anything in there. Most of what I change in the Import dialog box is by changing the import preset, which is very reliable because presets are already known to do exactly what I set them up to do.
Another reason is that working in the Import dialog box is slow. In part because nothing is actually cataloged yet, so no image caches have been built, so there is little performance optimization available. Because none of the images are in the catalog yet, they are being read off the camera card which is probably much slower than the SSD in your computer. It can be much slower to go from image to image in Import than in Library.
A third reason is the limited number of tools and shortcuts in the Import dialog box (because it’s just for importing). Several of the keyboard shortcuts for fast selection/deselection of grid images do not work in the Import dialog box. Some of the keyboard shortcuts that are available in the Import dialog box are range selection (Shift+arrow keys), loupe (E), grid (G), and some but not all of the zoom shortcuts. The rest are not available. In addition, many keyword acceleration tools (such as keyword sets, the Keywords and Recent Keywords panels, the Painter tool, and all their keyboard shortcuts) are not available in the Import dialog box. If you could apply keywords to selected ranges in the Import dialog box, it would be a crippled experience anyway.
When you put together all those concerns about performance, unavailable keyboarding features/shortcuts, and not being able to save changes until the import actually happens, the Import dialog box is sort of a worst-case scenario for getting real work done. Better to wait until after import.
Sure, in the Import dialog box it makes sense to deselect some images that you definitely don’t want to import, and it also makes sense to add keywords when they will apply to all images in that import batch.
But, keywording after import in the Library module can be far faster and more efficient than in the Import dialog box, because you have the full range of specialized keywording tools, presets, and keyboard shortcuts, the full Undo stack, and the advantage of cached previews to accelerate image-to-image navigation.