Brad, we have endless patience for our brethren truly seeking THE TRUTH. Answering questions posed in this forum is a great way to affirm and strengthen scriptural knowledge. If I don't know the answer (not that I am the authority here by any means), someone else here most certainly will, but if there is no definitive answer, no one will spit on your head and tell you it's raining.
The answers to your questions will be mainly based on conjecture, on my part.
Does this statement still protect the Jew today?
I don't believe this does. This was Yahweh's way of ensuring His means of punishment for the Israelites. If Yahweh allowed the destruction of Cain prematurely, there would be no physical manifestation of evil for the Israelites to struggle against and prove their worthiness to Yahweh.
Are we Israelites simply supposed to shun the Jew and not associate with the Jew, instead of harm him?
Absolutely. When Yahshua returns, we will be His battle axe. Until then, we are instructed to come out of and be separate. I don't think, however, that this command is a direct result of Gen 4:15 or Gen 4:24. There are many instances in the Old Testament where Yahweh gave us the chance to wipe them out, but we disobeyed. He reiterated to us that these 'people' would plague us if we didn't eliminate them.
Is this the justification that Pissrael uses today when they say they will kill (X) number of Palestinians or Lebanese for each Jew killed by Palestinians or Lebanese?
If this is the case, it is a justification that they keep to themselves. They do not want us or the other races to know that they are the progeny of Cain. If they admit this justification, they are openly invalidating their claim to OUR heritage.
Later on, in Genesis 4:24, the son of Cain named Lamech uses this to justify the killing of more than sevenfold who did him "harm" (even though they never slayeth him).
A fuller account concerning this is found in one of the apocryphal books, the Second Book of Adam and Eve. Now, you have to be careful about the information imparted from the apocryphal books. Many have added text and the contents cannot be confirmed, but in this instance, Chapter 13:1-18 serves it's purpose in answering your question.
Lamech was the great, great, great grandson of Cain, who was also blind. It was Lamech who mistakenly killed Cain, at the behest of a young shepherd who was with Lamech tending to Lamech's cattle. Lamech was brought to believe that Cain was a wild beast or a robber, and was directed to aim and shoot his bow and sling at Cain by this shepherd. When Lamech realized it was Cain whom he slew, "in the bitterness of his regret" (I personally think it was anger induced from the knowledge that Lamech just brought the curse of Cain upon himself), he slew the young shepherd also. This is what Gen 4:23 is referencing: "...For I have killed a man for wounding me (possible reference to Cain disturbing his herd of cattle, or to the idea that all of Cain's progeny have been "hurt" by him), and a young man for bruising me; (possible reference to the shepherd's mistake of Cain's identity, thus bringing the curse of Cain upon Lamech, who obviously was aware of that curse)" All this incidence did was verify a continued curse on that bloodline.
I hope this answers your questions.
Yah Bless,
Matt