Hi Harish,
1. The power-up sequence is configurable by OTP, meaning each regulator is assigned to a slot by OTP configuration using the dedicated OTP bits. You can find more information in the FS26 datasheet, chapter 15.6.1 “Regulator power-up sequence”.

2. The LBIST verifies the correct functionality of the safety logic monitoring and the Fail-safe oscillator monitoring mechanism.
3. Depends on which LDO is used to supply VDDIO and its OTP slot configuration.
Best regards,
Tomas