1. it's considered an unremovable scale. The "scaling" will last on that ninja until that ninja dies, iruka's status of dead or alive doesnt affect the scale.

2. the scale is 15% of that ninja's base stat. So Kushina will get 15% scale from Kushina's base Attack and Ninjutsu, Iruka will get a 15% scale from his base attack and ninjutsu

3. i just answer above.

also... your description of iruka Y+1 passive is a bit wrong... it should be like this: before Iruka's first action each round, gives Konoha units a shield based on 25% of Iruka's resistance, while also increase their attack and ninjutsu by 15% (the attack and ninjutsu scale can stack to up to 5 times)