[nyanpasu64]

I heard that pianos have stretched harmonic series due to string tension/weight/something, so piano tuners actually have to tune upper notes higher and lower notes lower, while ensuring various harmonics interact well. There's quite a bit of art to it rather than pure numeric ratios (which may or may not be possible to encapsulate in ML).

[pclmulqdq]

Yes, this is correct for pianos. I believe it is actually due to the thickness of the strings varying a lot across the instrument.

Most piano tuners today use a tuner for a middle octave and then tune the outer octaves by ear.