The Last Boss has 4 Mobs near him, 2 on each side (not the ones he spawns). Do those mobs need to be killed before the Last Boss, or can I ignore them (leave them and not kill them) and just kill the Last Boss? Killing the Last Boss ends the dungeon, those 4 Mobs do not have to be killed. But I'm not sure if maybe that's against the rules to not kill all the mobs in the dungeon.

You can just ignore them, there's no rule requiring you to kill all mobs.