I would fain have kept (εβουλομην κατεχειν). Imperfect middle and present infinitive, "I was wishing to hold back." Again from the standpoint of the arrival of Onesimus.

In thy behalf (υπερ σου). So "in thy stead," "in place of thee."

He might minister (διακονη). Present active subjunctive (retained after εβουλομην) with ινα, purpose continued, "that he might keep on ministering."