To your questions -
1. The device may or may not be able to recover, as the hibernate circuits may not be reset (BOD) and can result in corruption and impact the behavior. Hence it is advised to turn off Hibernate if the power supply is not stable.
2. It would be better to add an external supervisor, if low (hibernate level) power is desired.
Meenakshi Sundaram R
Thanks for your answer! I have another idea:
Can I use the low power comparator to measure the supply voltage and if it drops below 1.8 V then the comparator will wake up the system from Hibernate with an reset and the BOR will be activated again?