It's true that Paul's letters show variation in the timing of the work of Christ. I think the best explanation of this is simply that Paul understands the work of Christ as a process begun but not yet finished. Therefore the variations are a reflection of Paul's varying focus throughout his letters.
For example, in 1 Corinthians 15.22-28 Paul is responding to a specific question from a local church. In verse 12 we read this:
But if it is preached that Christ has been raised from the dead, how can some of you say that there is no resurrection of the dead?
The question is about the general resurrection of the dead, which obviously for both Paul and his readers is a future event. And (possibly because of the passage of time) some church members are starting to doubt or deny that the resurrection of the dead will happen. Paul's response is to argue that the resurrection of Jesus is the guarantee of the resurrection of the dead. That's the point of verse 20:
Christ has indeed been raised from the dead, the firstfruits of those who have fallen asleep.
The firstfruits metaphor comes from the OT ceremonial system, where the firstfruits was a thanksgiving offering from the initial portion of the harvest. The people would bring their offerings as a sign of trust in the God who will ensure the full harvest. In the same way, if Jesus truly has been raised from the dead then this becomes a sign that all who trust in him will at the right time be raised too.
So when does Jesus' rule take place? Paul's approach here is that it will not be a complete rule until the final resurrection, because although Jesus himself is already risen, the work of Christ is to restore the whole of creation, and that won't happen until the final resurrection when all things will be subject to him.
When we turn to the second text, Ephesians 1.19-23, we find a different focus. Here the resurrection of Christ is a past event, as in 1 Corinthians; but in contrast to 1 Corinthians the ascension and rule at the right hand of God is also described as a past event. How do we respond to this contrast?
The context here is Paul's prayer for the Ephesian church. He prays that they may know the limitless power of God, and as the key illustration of that power he points to God's act in raising Jesus from the dead. But this is not just from death to life, but from death to glory, from the lowest point to the highest point. By this illustration he is assuring the Ephesian readers that nothing is impossible for God.
If we take these two texts in isolation it is certainly possible to see a tension or even a contradiction between them. But the overall context of Ephesians leads us to a different conclusion. Although Ephesians 1.19-23 by itself may read as if the work of Jesus is a fait accompli, the letter as a whole shows the same process of restoration as in 1 Corinthians. Consider these examples:
In Ephesians 1.13-14 the Ephesian believers experience the same past and future tension as in 1 Corinthians. On the one hand they are already "in Christ" and in possession of God's Holy Spirit (verse 13). On the other they are still awaiting the final receipt of their promised inheritance (verse 14).
In Ephesians 2 Paul can describe the believers as already raised to life in Christ. Not only that they are alive, they are raised to heaven and seated with Christ at the right hand of God (verses 4-6). All this is set in the past tense. It is fait accompli. And yet when we get to chapter 4 we see the other side of the coin. The church which is already reigning in heaven needs to work to make that real on earth. Paul needs to urge them to live out their Spirit created unity unity (verese 1-3). They must use their Christ given gifts to build the church (verse 7-12). The building metaphor points to a process, as does the goal of being like Christ (verse 13-16). That goal is still in the future as Paul writes.
The final appeal in Ephesians 6 is to put on the full armour of God and to keep praying (verse 10-20). Why? Because we are still in a spiritual struggle (verse 12).
So in summary it's simplistic to say that in Ephesians Paul writes of Jesus' work as a fait accompli. A careful reading of the whole letter shows the same kind of tension and balance between Christ's completed work and the application of that work in the church and the world that we see in 1 Corinthians 15. It is finished. It is not yet completed. Both are true.
