Is Satan already under Christ's feet since Jesus rose from the dead?
No. Although Christ rose victoriously over a defeated Satan, Satan is still the god of this world, still destroying lives. Christ sits at the Father's right hand until his enemies are made his footstool, so Satan is not yet under Christ's feet.
Les explains that God revealed the coming seed of the woman would deal with Satan's powers and crush them, which was accomplished at the cross as far as the potential. But even after Christ was raised from the dead victoriously over a defeated Satan, Satan is still the god of this world, still destroying lives. Therefore he is not yet under Christ's feet. Christ is to sit at the Father's right hand until his enemies are made his footstool. Hebrews chapter 1 shows Christ has accomplished the work of the cross and risen from the dead, and this connects with the discussion in 1 Corinthians 15.