1st scenerio, kushina although only scale nin, but her damage is tai/nin... so yes the damage outcome will be different if the opponent (in this case Kakuzu) have any defensive mechanism (in this case scaling of defense)

2nd scenerio, you have to consider 4 things: damage done, scaling of Kakuzu, scaling of Minato, and the tag debuff.

if at the beginning of the round, edo minato and kakuzu has no scaling, plus no tag on kakuzu... damage will be approximately 1.8k (for example)

at the beginning of round 5, edo minato has scaled, also kakuzu has scaled, and the tag debuff is there to reduce 30% of kakuzu's defense... so... can the damage dealt by edo minato still remains at around ~1.8k damage? yes it can.

so i see this as not a bug... just some fun experiments using debuffs and scalings.