A small question about rules

[Epo]

Player A dies to player B in a valid PVP encounter in system C. After the pew is over, player B leaves, so according to rules, player A can return to system C as the system ban is no longer in effect.

What happens if:

a) Player B returns to system C before the two hours have passed. Can he engage player A again, or not?

b) Player D, who wasn't present in previous encounter, when player A died, enters system C. Can player A engage player D, after returning from dead after player B left the system C?