Multiplexing PT100 with CD4051
The Texas Instruments datasheet of the CD4051 isn't very clear about the maximum current. My conclusion about violating the "Maximum Input Current of 1 µA" was wrong, that's the maximum leakage current.

The HEF4051 is a similar chip from Nexperia (former NXP/Philips) there the datasheet is more clear: the maximum current is 10 mA so that 1 mA is OK.

However the high on resistance will still prevent you from reaching the required accuracy. The on resistance will vary over almost anything like supply voltage and temperature.

What is possible is to use a current source for each sensor and then only multiplexing the resulting voltage to the ADC input.