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A118 It should work as you've said, but I can see someone arguing the other way as well. You definitely wouldn't get additional damage from cold for the Vulnerable All. Note that Radiant One's damage is when you deal damage, and it's extra damage- meaning that any additional damage instances, such as from Assassin's Shroud or Firewind Blade, do get the Radiant One damage again, and would ping the vulnerability again.

A120 Yes, because you are making the attack through the creature (Rules Compendium on summoning).

A121 Yes. Again, you make the attack through the creature.

A122 Unclear- expect table variation. I'd say yes, but I could see someone saying that the creature gains the feature rather than the power.