1) Celestines FNP is not a persisting effect so stops as soon as the last Geminae dies rather than at the end of the units shooting that killed the last geminae?
Yes. Lifewards is not a leader ability and therefore does not persist until the end of a unit's attacks are completed.
2) demonifuge mortal wound ability can be activated without disrupting actions
Holy Judgement is not a shooting attack if that is what you are asking.
3) assuming your not playing with the WTC FAQ the triumph of Saint Katharine aura can be used to perform multiple acts of faith and replace both dice on a charge
No. While a unit can use any number of Acts of Faith while within the Simulacrum of the Ebon Chalice aura, you can only use one Act of Faith on any specific roll. The Acts of Faith rule clearly states you are only allowed to replace 1 of the dice on a multiple dice roll like Battle-Shock or Charge rolls.