This might help some. Biblical and Jewish Traditional Beliefs About Purgatory.
As for not knowing for sure, that’s not really a big deal. The Bible says, “39: On the next day, as by that time it had become necessary, Judas and his men went to take up the bodies of the fallen and to bring them back to lie with their kinsmen in the sepulchres of their fathers.
40: Then under the tunic of every one of the dead they found sacred tokens of the idols of Jamnia, which the law forbids the Jews to wear. And it became clear to all that this was why these men had fallen.
41: So they all blessed the ways of the Lord, the righteous Judge, who reveals the things that are hidden;
42: and they turned to prayer, beseeching that the sin which had been committed might be wholly blotted out. And the noble Judas exhorted the people to keep themselves free from sin, for they had seen with their own eyes what had happened because of the sin of those who had fallen.
43: He also took up a collection, man by man, to the amount of two thousand drachmas of silver, and sent it to Jerusalem to provide for a sin offering. In doing this he acted very well and honorably, taking account of the resurrection.
44: For if he were not expecting that those who had fallen would rise again, it would have been superfluous and foolish to pray for the dead.
***45: But if he was looking to the splendid reward that is laid up for those who fall asleep in godliness, it was a holy and pious thought. Therefore he made atonement for the dead, that they might be delivered from their sin.***” (2nd Maccabees 12)
If one cannot know the state of another’s soul when they die, then how can you possibly pray for people in Purgatory? What if they’re in heaven or hell? Isn’t that just sort of grasping? A “just in case” prayer?
What’s wrong with “just in case”? If you feel led by the Holy Spirit to pray for me, “just in case” I might need special graces from God to deal with something, do you think that either God or I will not be pleased?