1. GK always have to take a psychic test to activate their force weapons instant death ability. This means that a regular unit of GK Strikers have to choose between casting hammerhand or activating their force weapons. This has nothing to do with the fact that halberds grant a +2 initiative. This is another special rule given by the weapon and is completely separate from it being able to deal instant death.
2. GK Strike Squads and similar GK infantry can only cast one power a turn. Hammerhand, Warp Quake, or activating force weapons.
3. No, there is no reason at all that a halberd would give an additional attack in CC.
These answers are all very plainly detailed in the BRB and your codex.
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