At the second schematic, I would change the cap to 2200uf, and also I would add 100uf parallel to the 50K resistor on the interrupt pin.

I would also add a BOR level or an endless loop, like a STOP command, immediately after the writing to the eeprom is complete.


When the power is lost, 2200uf will go down far after 100uf.
Once 100uf is down, you read zero at the interrupt pin and act right away to save to eeprom, and stop the PIC from doing anything else.