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.
"A great tactician creates plans.
A good tactician recognizes the soundness of a plan presented to him.
A fair tactician must see the plan succeed before offering approval.
Those with no tactical ability at all may never understand or accept the plan.
And when a mind is too deficient in understanding, the resulting gap is often filled with resentment."