1. Madara Edo Tensei released doesnt remove buffs that she received from herself (whether 8% def res or 6% nin crit)

2. how you use it will depend on who you are fighting. for ninja exams, if you think you lack more damage to kill enemies overtime, use the nin crit. if you feel you are dying too fast, use def res.

I would use def res only in ninja exam fights wherein Kushina is more of a support, and i need her as a support to break through debuff immune enemies

for DB or GNW, again it depends.

if issue is survivability, use def res.

if issue is you need to do more dmg overtime (and also heal better), then use nin crit

i tend to use nin crit more since my form enables me to survive longer (since i use sakura summer)

but if kushina just needs to survive herself without supporting ninjas, use def res.