I think we have to ask what Paul's idea of Resurrection would have been. He was a Pharisee, so believed there would be a Resurrection, but he probably believed as most did at his time this was to occur for most at " The End of Days" .
Generally Rabbis of his day envisaged a resurrection back to life on Earth if I remember correctly, they quoted the examples from Scripture where ones were formerly brought back to life, but bearing that in mind , how could Christ be the Firstborn from the dead ? I guess the conclusion above is right, he was the first to gain immortality, those previous guys all died in the end.
Paul has to develop these ideas, somewhat hurriedly, and way different to what orthodox Jews believed then, because he is convinced that Jesus , after his death, spoke to Paul from Heaven. And Mortals don't get in to Heaven, presumably.