Just had this come up a 1 hull z95 shot a kavil with R5-P8 with cluster Missiles could i have used R5-P8 to kill the z before it could fire the second lot of missiles?

No. Faq states directly you need to resolve all triggered attack possibilities before removing ship (confirmed by 1HP decimator using Vader to kill itself then (while technically dead!) using gunner then even vader second time).

I would have thought that because the damage is given external not from anything the Z95 or vader/gunner would do it would block the second attack. In the case of vader/gunner you are resolving that card abilities to kill your self, with R5-P8 he is interrupting you attack to triger giving you enough damage to kill the ship in between attacks

Given that cluster missiles specifically skip the need to declare the target on the second attack (in part to prevent players from attempting to justify the ability to attack two separate targets with their two salvos) - it seems to me that the second attack is uninterrupted even if the attacker is destroyed in the middle of that process.

There's a lot of tricky scenarios that come up in regards to ships continuing activated effects postmortem, but I don't know of any rulings off the top of my head where an effect, once activated, can be prematurely cancelled once triggered in the case of death.

Just had this come up a 1 hull z95 shot a kavil with R5-P8 with cluster Missiles could i have used R5-P8 to kill the z before it could fire the second lot of missiles?

Yes, you could have triggered R5-P8 and possibly destroyed the Z-95 before the second lot of missiles were fired. But it would not have stopped the second lot of missiles being fired. The Cluster Missile card has been triggered, so both attacks must be resolved. They are treated as two separate attacks for all abilities that can possibly trigger "after attacking" or "after defending" or "after an attack...", but they still get resolved fully regardless of whether or not the firing ship is still around for the second attack.