2 separate questions. Can a player play Comm Disruptor in response to negation? I believe it's allowable and pretty potent if you have take initiative.

If Jyn plays One in a million can my opponent play stealth tactics to add a white die to the attack pool?

Disclaimer: not an expert in Skirmish rulings.

I don't see an issue in cancelling Negation with Comm Disruption.

One in a Million is played when declaring an attack, during step 1, Stealth Tactics must be played before dice are rolled, i.e. immediately before step 2 of the attack. I don't see any problem. Stealth Tactics triggers after One in a Million.

RAW: An ability that adds dice to a dice pool can only be used immediately before step 2 of the attack.

This would imply stealth tactics has to be used during declare attack step as well. Would the timing be attacker effects first then defender effects thus the defender gets to add the white die?

RAW: An ability that adds dice to a dice pool can only be used immediately before step 2 of the attack.

This would imply stealth tactics has to be used during declare attack step as well. Would the timing be attacker effects first then defender effects thus the defender gets to add the white die?

Declare Target is step 1. Roll Dice is step 2.

Stealth Tactics must be used immediately before step 2, i.e. immediately after step 1.

The abilities do not have the same trigger, so timing conflict rules do not apply.

You cannot be 'while defending' unless you were declared as the target of the attack during step 1, so Declare target and all abilities triggering off of it must be performed before entering 'while defending'.

(However, "when you're declared the target of an attack" would use the same trigger, but performed after all attacker abilities with the same trigger due to the timing rules.)