The MCU pieces came, I tested them unsing the multimeter and the pad in the center does not connect to anything. So it is an NC pad. I guess optionally one can connect it to GND.
(Guess its used as heatsink)
Certainly it looks like the BOD levels are selected for 5,10 and 20 MHz and if you want to use a different frequency then there isn’t an ideal BOD level and you’d need an external voltage monitor. It’s fair to say that most devices will perform properly somewhat below the minimum guaranteed level - bear in mind that this is (hopefully) tested in the factory ...