Xabre, the only tools I have to do that are the ones that I've already laid out about models making attacks, and how all attacks are made with melee weapons. A relatively simple question that i pose then, for the argument that they just don't get modified by what weapon your wielding... then what weapon are they modified by for the combat familiar? All attacks (and specifically attacks) are made with a melee weapon. It's why you can't use a shooting attack in close combat. Saddly, I actually can't find a specific rule anywhere that actually says attacks are made with weapons. It seems incredibly implicit, but of course not having a rule i can post to for this one hurts my case.
What I can say on that note, is that free hits are not attacks and therefore are not bound by needing a weapon (vector strike, hammer of wrath, etc), and models without a weapon of the melee subtype are assumed to have a single CCW. These facts combined make it something that can be easily agreed on, though. If you want to debate that part, I'd be happy to.
Anyways, assuming that all attacks must be made with a weapon of the melee subtype in CC, then what weapon do you use for the combat familiar, if not the one the model who purchased it is always swinging with? If you made them as they start out, which is str 4 ap -, you're using a CCW (it has no modifiers, and no special rules). Well, what if it's huron blackheart doing that? Huron doesn't even have a CCW on his statline, and he has a weapon of the melee subtype, so he can't just pick to use a free CCW anyways. And, even if he could, it would be violating the "more then one weapon" subsection of the weapon section in the rulebook, because he's now mixing weapons for his attacks, AND he's doing it with a weapon he doesn't have.
In summary, the crux of my argument (put simply) is that the rulebook says that you have to pick a weapon in close combat, and EVERY attack that you make has to be modified by the weapon you pick. The combat familiar says you make attacks. Therefore, you apply all rules for making attacks for that model, such as using his WS, Init, and his weapon modifiers for those attacks, because they follow the same rules and wording out of the BRB.