This would be sort of a retcon, but a possible way of bringing Kirk Prime back would be to simply say that when Picard went into the Nexus and brought Kirk back, Picard actually brought back the "echo" of Kirk's essence in the Nexus, not Kirk himself. Kirk Prime could have actually been rescued, or left the Nexus prior to the events of Generations, which turns out to be the events in Star Trek 13. That might be a little hoaky,but at least it finally puts to bed the Nexus problem, and fixes one of the most unpopular scenes in Star Trek history. Basically, the Kirk we saw die, was just an "echo." Kirk Prime is then free to show up in the alternate timeline.