Assuming you mean the buff passive.

"For every 10 chakra a ninja from your lineup uses, increases such ninja's Attack by 8% and Critical by 3%. (Stackable, this effects persists for the whole battle)"

So, to answer the first: If you have a ninja that uses a 40 Chakra mystery attack, that ninja will gain (40/10 x 8%) 32% Attack and (40/10 x 3%) 12% critical. This also stacks and persists. Meaning it cannot be taken away by a ninja that removes buffs/debuffs and it can be done multiple times. So if you use an Edo Minato mystery every round, it will gain 32% attack and 12% critical every time it mysteries.

Viable with Sasuke [Rinnesharingan]? Not really. Her chase will not get used by Sasuke at all unless it is with your other lineups, and the main reason to use Sasuke is with his passive skill trial where he has a 0 CD/Chakra mystery when you have more than 70 chakra. Which the Kushina's passive will not be used because it doesnt use any chakra.

Did you test it out or do you just assume it works that way? Thanks alot anyways!

Assumption, I could ask the one person on the server who has her to test it though.

no need to assume.

it's confirmed and all correct, except for 1 thing

If Kushina got erased out of the battlefield (from skills which prevents revival like Madara GNW chase, Masked Man mystery, Tendo chase +2, etc...), not only does the passive won't be active anymore, but the scales that the units has gotten through her passive is gone as well.

For example, Edo Minato mys 2 times from r1 to round 3, gaining 82% Attack, 24% Critical and 24% Combo scale (64% Attack from Kushina and 18% Attack from his own chase), but at round 3 Kushina got erased by Masked Man's mystery

after Kushina got erased, Minato lose the 64% Attack and 24% critical scale.