Tom, I believe the answer to unlimited atonement is found in Romans 6. You have to think about it like this, what are the wages of sin? Death. Did Jesus die? Yes. So he paid the penalty for sin.
When we trust Christ, we are baptized into his body by the Spirit, we literally become one with him. So when Jesus died and paid the penalty of sin, we died with him. This is what the scriptures actually say.
Rom 6:3 Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
5 For we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
6 Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
So, when we trust Christ we are born again, we are baptized into Christ, we are one with him.
1 Cor 6:17 But he that is joined unto the Lord is one spirit.
When Jesus died, if we are in him, we died also. The penalty for our sin has been paid.
Does that mean Jesus's payment is limited? No, anyone can trust Christ if they choose to do so, the payment for their sin has been met by Jesus, Jesus died. No one is prevented from receiving this payment except by their own unwillingness to trust Christ.
If they refuse to receive Jesus, then they must pay the penalty for their sin, death.