I have a question about the order in which inputs or abilities are processed in PvP. I'm not new to PvP, it's always been my focus in the MMO's I've played, but FF PvP has me a bit confused on some outcomes. At the best of times, it feels like the games combat system isn't really quick enough for real PvP and you shrug it off. At the worst of times, it feels like I'm PvP'ing on dial up. So I'm going to lay out a couple scenarios and let me know if the outcome I expect is possible, or if I should just do something else all together.

I play a BLM in PvP, with a tank in the party I have 10,680 HP. Enough to survive a melee LB with a tiny bit of breathing room.

Scenario 1 -
Stealth ninja sneak attacks, stuns, LB's
I purify (remove stun), mana wall (20% max HP damage absorbed)
The LB cast finishes, I take 9600 damage and die or am close to death

If I am trusting the cast bar that I see for their Raw Destruction, then I cast mana wall in time and should have taken much less damage.

Scenario 2 -
<any melee class does a closer with or without stun>, then LB's
I cast Night Wing
Target goes to sleep well before Raw Destruction's cast is finished, but I still take full damage.

Scenario 2 is extra frustrating because I also try using Night Wing to save others from eating a melee LB but it simply seems impossible to interrupt it.... But...

Scenario 3 -
I'm a ninja, I sneak attack, stun, and LB <any class with a stun>
<any class that has a stun> stuns me, my cast ends, they take no damage

It feels to me as though a stun is the only status effect that the game processes quick enough to stop a melee LB cast.

General range/LoS scenario -
Melee casts LB on me, I run though them so I'm behind them, I Aetherial Manipulate myself 25y away, I Shukuchi 20y away, I repelling shot 10y away, none of that matters. I end up taking the LB damage.

This feels very WoW like to me, where the rule is ultra basic "You were in range when I hit the button, doesn't matter where you are 2 seconds from now, it is going to hit you". However, that same rule doesn't apply to regular abilities. I acknowledge that there is some flexibility if a target is just ever so slightly out of range of my cast by the time the cast finishes, but other times if they run far enough away or break LoS before the cast finishes, my cast is interrupted. Turning around while stationary and casting on a target within range breaks my cast a lot of the time. Shouldn't the same rule apply to LB?

I have a feeling that with gear, time, and a new level sync cap in the future, that melee LB will be less of a danger than it is now - meaning that anyone that doesn't have sta acc's is within 1 shot range - but for now, I'm wondering what I should be doing about it because my inputs and abilities seem incapable of doing anything to that Raw Destruction cast bar.