Great backstabbing (high damage) weapon

You do not necessarily need a Cracked Blue Eye Orb to advance in the covenant. By wearing the Guardian's Seal, a ring that you get upon joining the covenant, you can be summoned by members of the Way of the Blue, a different covenant, when they are invaded. Of course, you still need to be human to be summoned.

As for dispensing justice... yeah, you're kinda out of luck there. Games in the Soul series are rather more catering to offensive PvP than defensive PvP or "white knighting". However, there are typically hotzones for PvP in each Souls game. As Dark Souls 2 has only been recently released, the hotzones probably aren't determined yet. Once more people play and get used to the game, these zones will be recognized and you'll have a better time of finding an appropriate area to "deal justice".

This was known as a phantom parry, it happened when walking backwards before doing the riposte attack and attacking just before leaving the riposte range. This affected multiple, but not all weapons (The normal dagger is an example for a weapon it was often used for).

The game would essentially ad a normal rb/r1 attack to the riposte, adding to the overall damage.

I thought it was fixed a while ago though.

For more information on the technical stuff check the video "Dark Souls 2: Technique - Phantom Ripostes & Backstabs" on youtube.