It's not about what you can do to avoid the scenario. It is possible, that's the only point. I dont want to give ideas here but what if ppl decide to run double op healer + sorc healer with a tank, just looking for that situation?
It's simple logic when you think about how acid works. No matter how much total hp you have, it drains a certain amount in percentage, making everyone die at the same time given that you have full hp, get hit at the same time by acid and dont get any other dmg.
About usage of barrier in acid: I think it should be usable, because how to win acid is simply get hit by it later and do more dmg/avoid getting more dmg (Player dmg!). Acid dmg ticks on barrier and it should also work on the absorb after barrier.
It's the same with phasewalk. It's just a good usage of the cooldown to avoid player dmg, but acid dmg ticks on the assasin like on all others so it's ok in my opinion.