Thank you very much @grosgood for your detailed explanation/tutorial and everyone else as well for your help.
Indeed, following the rules @grosgood layed down, I could get a 100% identical result between Qt and the -inpaint command. I might make a custom command that behaves like Qt but takes 2 images as input (the one to inpaint and the mask) instead of selecting by color, but at least I understand how it works much better now.
I hope that future development of filters won’t obscure the meaning of sliders as much, as artists are used to work with clarified types of numbers, whether they are pixels, percents, multipliers or what have you. An informative interface helps tremendously in getting quickly to the desired result, even if we don’t necessarily have a complete knowledge of the inner workings of the algorithm that produces that result.
But hey, most important is to have great people who code powerful features and other great people who can help explain them if needed. And that’s a double check on that!