About Potential Status what is >> " 5% chance to ignore 40% dmg when hit " ?

In that case, yes, I'm quite sure they are the same.

The wording is quite odd, I agree. :x

Exactly what it says. You have a 5% chance of taking 40% less damage when you get hit. Different from weapon def ignore because that means you ignore 30% of a mob's defense when you attack (thus dealing 30% more damage).

Unsure about the stacking, someone else probably knows this...

You were right the first time (see: second quote). I have this on an equip and you should have seen evidence of it occurring, or at least compared it to instances where you might have seen your mythical 40% damage ignore.