Franck, Your post is very well done. I would suggest that your disappointment should be balanced by cost and workflow. "For previewing the images and finding the good ones" is not as simple as it sounds.
If all you want is to see if it is critically sharp, then nothing is quicker and easier than using something like a 40X microscope (Carson MV820) on a lightbox.
What you can optically verify that is sharp with this will be what you can expect from pixel peeping a Coolscan 4000dpi scan. It is also an 8X loupe for general viewing.
As you can see from your results, doing a quick preview of color negatives is a completely different matter altogether as using a celphone/dslr will require considerable work. Having scanned over 30,000 frames of various film types/brands with various methods (flatbeds, dedicated, minilabs, Imacons, drum scanners & dslrs), nothing beats the default neutral settings of Coolscan+Nikonscan.