32.32 fixed point? That's even worse!

Wrong. In the general case (long samples), 32.32 fixed point provides better precision than 64bit floating point.

If you don't use integer resamplings, this: (n*x)/y is the right operation. n is current sample, x is old sample rate, y is new sample rate.

And again you are trying to solve the general case by providing an example for a special case. The sample playback rate might change at a point n that does not allow the fraction to be reduced. You cannot devise a LCM for all possible denominators, you either have to choose a time base and round, or store arbitrarily large integers.

Do you think it's not noticeable?

Yes, it is not noticeable. The precision error in pitch is less than 1 cent even for extreme resampling ratios. This is not noticeable:

https://en.wikipedia.org/wiki/Cent_(music). Stop thinking in the time domain, this will get you nowhere here. And if you do not know what I am talking about, you absolutely will have to do the required background research, even if it takes you multiple years.

The case you describe is AGAIN a special case. The general case has varying pitches where nearest neighbour interpolation is by definition already broken way more than a single sample phase difference or rounding error.

If you for whatever reason need sample-accurate nearest neighbour resampling, and do not care about varying pitches whatsoever at all, use some software that actually tries to solve that use case. OpenMPT surely does not. Your usecase is absolutely meaningless in practice.

However, all of that is even completely irrelevant if you take a closer look at what the sample playback rate actually is supposed to be in practice. It is not an integer and not even a precise fraction. In the nowadays overwhelmingly common default case of equal temperament scale tuning, all but one of the note playback rates per octave are non-integer and non-rational. Your idea is already flawed at the preconditions. There is no precise fraction to even work with in the first place.

How can I spend (more than) ten years when I'm 12? Serious? Use your common sense.

So, it should not be that surprising at all to you that people who have worked more than 10 years in the field actually know things better than you, right? You have demonstrated multiple times that you did not think through your ideas thoroughly (which is totally fine, given the age), but when we tell you multiple times that your ideas are flawed, you absolutely have to step back somewhen and accept that you do not have the required background knowledge yet to fully understand all aspects (which again would be fine). But then, you not only expect us to explain things in a couple of forum posts that would normally take years to learn, and additionally also keep insisting on the same flawed concept over and over again? How are we supposed to solve that? By now, you are honestly only wasting everyones time, including your own, here.

Also, kindly asking how and why the mixer and resampler is implemented the way it is would probably have been more helpful for you and everyone else, rather than repeatedly insulting the developers and insisting on impractical ideas. Please reconsider your attitude.