You stated Juggernaut has never died when he in fact has and he confirmed so himself in a Spiderman issue which makes you wrong. Regardless of who killed him he HAS been killed, and no, Eternity cast him to Oblivion's realm where he grew old and died. Who has killed Hulk?
In New Exaclibur when he was on the team. There was an arc involving him that tried to point out some of it.
It wasn't well written in terms of a retcon cause it didn't go into great details but it turns out that Xavier was Cyttorak's first choice and that somehow connects Onslaught to the gem or at least mentally drawn to it.
It's possible that somehow allowed him to bypass Cain's durability and draw the gem towards them.
cain hasn't been used in any capacity since his last appearance in dark avengers #183, when he ventured out to do his own thing with his 'juggernaut-power' fluctuating in and out.
"This bold vision of the future requires a vast military dedicated to upholding the enactment of universal laws which are necessary to ensure galactic harmony.
In truth, these actions are taken merely to protect us from those who would invade, enslave, or foment political dissent, and to punish accordingly any who engage in such acts.
Look on our new military not as trespassers or interlopers, but as gatekeepers, here to shore up the Emperor's vision of a pacified and prosperous galaxy."