Whom I would have retained with me, that in thy stead - “That he might render me the service which I know you would if you were here.” The Greek is: “for thee” ὑπὲρ σοῦ huper sou; that is, what he should do for Paul might be regarded as done by Philemon himself.
He might have ministered unto me - He might have rendered me assistance (διακονῇ diakonē); to wit, in such a way as one who was in bonds would need.