Question B4:
The manufacturer specifies the maximum clock frequency at which they would guarantee correct operation of the device, and the associated operating voltage. In the case of the 328p, the max clock frequency is 16MHz @ 5V. (EDIT: it's actually 20MHz @ 5V) This frequency is reduced as the voltage is lowered. You can try running the 328p at a higher frequency but correct operation is not guaranteed.
Question B5:
There is a graph of frequency vs voltage in the datasheet. That will tell you the manufacturers recommended max frequency for the supply voltage you are using. My interpretation of that sentence is that the external clock frequency can be higher than the recommended frequency as long as you divide it down internally so that it is at or lower than the frequency from the graph.