And, even using your logic that you go by the activation roll, in the multiple assault the initial model wounded does not have a Tail, so when the power is activated there wouldn't be any members of the other unit targeted. You sound like you would try to argue that you would also get a save against a unit of Eldar that wounded the unit because a Warlock's Enhance power let the Warlock make the hit that wounded the SW. Like the wording for Nemesis Force Weapons, it's not directly targeting that Space Wolf unit, so you don't get to make saves against powers that aren't targeting you. Some things are indirect - like Enhance affecting the Eldar unit (just happening to make them hit better and quicker) or Grey Knight's Nememis Force Weapons having the power target all the weapons instead of targeting the person(s) hit.
The multiple initiative example in the GK Dex confirms this for me - for that guy going first, his Nemesis weapon functions EXACTLY as per a regular Force weapon. So in that case, it's a completely different rule? No, it's just that the roll for that individuals use of the psychic power has to be made then, and as per Grey Knight rules, that roll counts for the whole unit.
Nothing in the GK Dex supercedes the Main Rulebook on how the instant death works, or how it is as a psychic attack. The Force weapon, or Nemesis simply confers the power upon it's bearer.
In the multiple assault example you give, I would say that the unit with the wolf tail talisman would get to save against any instant death against it's members, and a successful save would dispel that individual effect of the attack that was saved against. The unit without the WTT would still suffer the effect without a save.[/QUOTE]