As far as I remember, Explosive Heart Packs was always in events for a long time before his Skillbreaks came out (I'm pretty sure about this because I pieced my Kakuzu from the coupon events for the sole purpose of having him with my Konan [Angel of God] team) and after the release of Skillbreaks, that's the time where his frags were placed in the Recharge Events. Then for the time where his Skill Trials came out, a couple of weeks or months before it got released, it was already back in some coupon events and even placed more continuously in coupon events after the Skill Trials comes live in the game.

Now about the question of "Will Kushina Fragments will be an Ingot exclusive for some time", I don't have any idea or information about it. All we can do is wait for now.