I believe the logic is that they end up in the hell-realms for the short term (even though, by convensional terms, this is supposedly still a very long time!) where it is difficult to practice the Dhamma under the weight of one's kamma, whereas in the long-term there may be rebirth back in the human realm (or another relatively comfortable realm) where the Dhamma can be learned and enlightenment achieved.
As for the inevitability of enlightenment, I don't know that the Buddha actually discussed it.
As for omnipotency, no. The arahant has achieved the permanent cessation of suffering through the eradication of greed, aversion and delusion. This however, from the Buddhist perspective, is a far more desirable outcome than being able to achieve all those supernormal feats you speak of.
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"The uprooting of identity is seen by the noble ones as pleasurable; but this contradicts what the whole world sees." (Snp 3.12)
"To argue with a person who has renounced the use of reason is like administering medicine to the dead" - Thomas Paine