Unless I'm mistaken, Tenacity Down is not resistible. This is because TenacityDOWN is a counter to TenacityUP and to generally high tenacity. If you have to land TenacityDOWN against someone with TU or high T, then it ceases to function as a counter.
So, again, assuming I understand things correctly, which I might not, the TD will land because it cannot be resisted and "resists all debuffs" only resists all debuffs that the game allows a chance to resist. But since Padmé's ability causes her squad to resist "all" debuffs, landing the TenacityDOWN won't help any ally of HK land any other debuffs.
Either those other debuffs are also not resistible, in which case the TD doesn't matter, or they are, in which case they will be resisted automatically ... and thus TD doesn't matter.
The only way the TD can possibly affect the game at that point is counting as a dispellable debuff that increase damage for things like Vader's Culling Blade or Boba's Execute or GeoSpy's special, whatever it's called.