Since Dante and St. John Damascene say Emperor Trajan was freed from hell, can souls in hell be saved?

That is impossible. The church has never taught, and never will teach, that a soul from hell can go to heaven; it is not in Scripture and not in tradition. So wherever this story is coming from, and Dante is one source, I can see why with him, but as for St. John Damascene, I would be very surprised to find anything of the sort in his writings. You can check that out if you want, but a soul going from hell to heaven would be an impossible thing.