From a theological view, is it possible for God to predestine that XYZ be saved, yet XYZ to reject predestination throughout their life?
Without any question any person who never believes in predestination can still be considered a saved and regenerate predestined Christian by a Calvinist. All that is required to be considered 'predestined' in the eyes of men, is that a person believes in Christ for the forgiveness of their sins. Anyone who truly 'confesses Christ' in this sense will be saved, regardless of how clear, or confused they may be on the surrounding doctrines of their salvation.
Knowing how Calvinists leaders have thought through history, I can assure you if someone added a requirement like 'faith in the doctrine of predestination' to be needed in addition to 'faith in Christ' they would have denounced it. It is a 'person' who died for our sin that we must put our faith in, not a textbook statement phrased by church leaders.
This is a common point of confusion for people who haven't been exposed to much Reformed theology, so let me try to state this briefly and directly.
The Calvinistic doctrine of Predestination fits into a larger framework of doctrines, all built on scripture and playing off of each-other. If you try to transplant the doctrines like pieces from a puzzle and fit them into a different puzzle, they will stand out and look wrong.
In particular, our understanding of Predestination goes hand in hand with our understanding of salvation as a grace bestowed upon us by God. Just like we believe salvation cannot be earned through doing good works, we believe that it cannot be merited by believing the most sound doctrine. Quite simply this means that even people who actually hold some WRONG beliefs about God and salvation are going to be saved just because God wills it to be so.
On the flip side, we do believe that it pleases God for us to know how deeply we are indebted to his grace and that the closer we come to holding and promoting sound doctrine the more we will be transformed to be like him. While someone who insists on believing that it was their personal choice to follow God may well be saved by God's grace, an area of their life is held hostage to a kind of pride that thinks they accomplished something. One who deeply understands that they did not merit their salvation through their own actions or beliefs is setting the stage for a more humble and thankful life of grateful service.
In other words, belief in the doctrine of Predestination isn't necessary for salvation, but we do believe it serves a purpose in the life of a believer.