That's a strong write-up, and I like your view on this topic. It's a much more eloquent fix for the cooldown than what I have thought up, and it's a bit more flexible in a spec's playstyle and rotation. I'd assume that the "avoided" tag in your ability description wouldn't apply to Dodge/Evasion, Force Shroud/Resilience, or similar effects? In other words, it can't ignore defensive cooldowns that are designed to have a 100% chance of avoided a certain attack?
Of course not. First thought was that it would be implicit, i.e. absolute immunity trumps it. I think there are a few things in the game that "can not be...", but are by total defenses.
A more descriptive phrasing could be:
"Increases the critical hit chance of your next Ambush, Series of Shots, Explosive Probe, Takedown or Cull by 100% and makes it unable to be intercepted, avoided or mitigated by abilities that provide partial protection."
Either way, exact wording would be up to the developers, if this change is done, but this is understandable enough. We don't need a cheat, just a counter to PvP tanking and "can't kill me" cycling of defenses. Not a hard counter either, it still only a moderate bit of damage once in a long while, but it could start the work toward getting Snipers back into competitive PvP.
Right now, Smash Maras can finish off tanks at otherwise survivable hp, and they can do it easily, repeatably, twice a minute, with their full ease of rotation and AoE power; AP PTs are high up there as well. Letting Snipers do that, but to the guarded player instead of the tank, only once in two minutes, damaging a single target only, requiring correct setup, isn't much. It's balanced, it fits their role, it doesn't change the relative ranking, just narrows the gap, adds a little more reason to consider the class for ranked compositions.
As a regular 15-30-second cooldown, it would secure a very fitting role for Snipers as "healer killers". A
s a 2-minute cooldown it doesn't, but it lets them to occasionally participate in putting pressure on the enemy team rather than merely send more damage against their heals.