I would fain have kept (eboulomēn katechein). Imperfect middle and present infinitive, “I was wishing to hold back.” Again from the standpoint of the arrival of Onesimus.
In thy behalf (huper sou). So “in thy stead,” “in place of thee.”
He might minister (diakonēi). Present active subjunctive (retained after eboulomēn) with hina, purpose continued, “that he might keep on ministering.”