Hello,
1. ADC calibration is only internal error reporting of TFF82. This design only provides external 40MHz crystal oscillator, is it related to this crystal oscillator? Or does it have to do with RF routing?
Ahh, ok now I see. I get it all wrong.
The issue is not from uC but from AFE.
The ADC calibration of TEF82xx is described in TEF82xx reference manual:
The details on ADC calibration are in chapter 4.7.1 ADC calibration

Since I am not familiar with RSDK background calibration, its hard to say, but I expect there must be some parameters set for calibration.
Could you share how you do the background calibration?
Apart from this, it is always good to submit a ticket at NXP web page as then you can get in touch with RADAR team directly.
Best regards,
Peter