abug I think you misunderstood slightly ...
Perhaps it would be better if you explained exactly what you want to achieve?
abug 'load_images_max_filesize' => 256000,
abug 1) Images below 256K will display a preview icon as the original image, while images larger than this size will generate a thumbnail.
Not necessarily. This option only applies if the thumbnail is NOT generated. It means "don't load images that are larger than 256 KB as previews". Or only load images that are smaller than 256 KB as previews. This option will normally not even apply when you are loading thumbnails. The reason this option exists, is to prevent extremely large images being loaded as thumbnails. For example, if you have 20MB images at 10.000x10.000 px, that don't have thumbnail, then you simply don't want this loaded as a tiny preview image (it will takes ages to load, consume all the browsers memory, and make scroll laggy).
abug 'image_resize_max_pixels' => 60000000,
abug 2) Images with a width exceeding 60000000 pixels do not generate a thumbnail image, but are displayed as regular image file icons
Yes, it means images with pixel dimensions (width * height) that exceed the value, do not generate thumbnails. This is to protect the server from attempting to resize images that it is not capable of resizing, or if the process becomes too slow or takes too much resources. Normally, it will then display an ICON yes.
abug In the demonstration image, any image with a width exceeding 10240 pixels cannot display thumbnails. I don't know why?
Probably because it exceeds the 'image_resize_max_pixels' => 60000000, ... Doesn't that make sense? 10240 pixels width * height (which I don't know), can easily exceed 60000000 ... For example (w)10240*(h)6000=61440000, which exceeds 60000000. This image is massive, and you need to make sure your server is powerful enough to resize massive images. That's why this setting exists. You can try to increase the value ...
The image_resize_max_pixels is not really related to load_images_max_filesize.