Sure, atk down does mitigate it, but adds another 1.2k to the base value, so you're receiving 2.4k*(1-0.2)=1920 in case of 20% atk down instead of 1.2k. Again, that 1.2k is on dark, so in case you're running off-element, it becomes around 1.6k base value / debuff.
As for accs, their effects get a minor upgrade at max level like 1% DA or TA turns into 2%, but that's it.
They're in fact guardians by name. They refer to themselves as "守護者" which translates to exactly that.It's not a made up term.