First, there is nothing in your post to challenge the scriptural basis for the doctrine of purgatory. I am tempted to just leave it at that, with a remark that you do not understand Catholicism well enough to point out contradictions.
But since there is obviously interest on this thread in understanding Catholicism better, I will explain. It is true that generally no one knows who is saved and who is lost except at the end of one's life by the faithful himself. It is likewise not known if a saved soul is in purgatory or has already made it to heaven. Further, the very concept of purgatorial fire existing in the temporal domain measurable by time is a popular piety: it is helpful to think of it that way, but it is not a dogma of the Church.
We do not know if our prayers for the dead are applicable every time. The dead soul may be truly dead in hell, or in heaven and not requiring our prayers in either case. We pray because we want cleansing of saved souls, when possible, if possible.
We do have two kinds of relative knoweldge and relative assurance. First, we know of the salvific, transforming efficacy of the sacraments of the Church. We then know that one properly dispatched by the Church to his rest is on his way to heaven. Second, we know that the Church is capable of scrutinizing a particular life and upon evidence of miracles it worked as evidence of its santification, the Church can confidently canonized this man saint, who then will hear our prayers and pray for us in heaven.
Your puzzlement would have been well placed if we considered good works salvific alone. Then the disposition of the worker would not have mattered and the works being an objective fact could lead us to objective conclusion of the state of the departed soul. Works in life would be purchasing salvation in the afterlife. But of course works are salvific inasmuch as they draw our heart to Christ, and here is the great mystery, the mystery of the Beatitudes: blessed are those whose works, albeit objectively poor, are done in Spirit.
Are Christian beliefs like this, beliefs shrouded in mystery, superstition? Fools think so. But we see wintesses in heaven.
