Well, I would say it was not a "potential to save all" because God knew not all would believe. More like God ordained that all who believe will have eternal life, knowing who would believe, but not "looking down the corridors of time", because God's exhaustive foreknowledge makes such a thing unnecessary
Anyways, the fact that they did not believe does not mean that God did not give them grace to believe. The point of difference is resistable vs irresistable grace and going even further back, the basis of God's election. People act like Arminianism and Calvinism are a razor's edge apart, and they are in some matters, but in the conclusions they draw, it is like night or day. That is why the same people that try to say they are close also say Arminianism and Calvinism is an either/or proposition. But I disagree. To say it (Arminianism/Calvinism) is an either or is a false dichotomy, and ignores the fact that there are many shades of both, and also there are things in both that are speculative. JMHO.
Concerning PREVENIENT GRACE. If you just go by what the words mean, prevenient means "going before" so it is grace that comes before faith. Calvinists believe in a prevenient (before faith) grace, they say regeneration precedes faith. Therefore, it is a prevenient grace. Now, before Calvinists start telling me I don't know what I'm talking about, I perfectly know exactly what prevenient grace means, and why Calvinism is different, but I am only going by the actual MEANING of the term without the baggage attached to it.
To me, Prevenient Grace AS I UNDERSTAND it, is exactly the same as what Calvinists call regeneration. Except Calvinists see Grace as irresistable. I know perfectly well that my definition of Prevenient Grace is not the same as Arminians, but that is why I say I am neither Calvinist nor Arminian. Another reason Calvinism/Arminianism is not an either/or proposition, IMHO.