Yea (αλλα). Confirmatory use as in 2 Corinthians 7:11, rather than adversative.

The answer of death (το αποκριμα του θανατου) This late word from αποκρινομα, to reply, occurs nowhere else in N.T., but is in Josephus, Polybius, inscriptions and papyri (Deissmann, Bible Studies, p. 257; Moulton and Milligan's Vocabulary), and always in the sense of decision or judgment rendered. But Vulgate renders it by responsum and that idea suits best here, unless Paul conceives God as rendering the decision of death.

We ourselves have had within ourselves (αυτο εν εαυτοις εσχηκαμεν). Regular perfect of εχω, to have. And still have the vivid recollection of that experience. For this lively dramatic use of the present perfect indicative for a past experience see also εσχηκα in 2 Corinthians 2:13 (Moulton, Prolegomena, p. 143f.; Robertson, Grammar, p. 896f.).

That we should not trust in ourselves (ινα μη πεποιθοτες ωμεν εφ' εαυτοις). A further purpose of God in affliction beyond that in verse 2 Corinthians 1:4. "This dreadful trial was sent to him in order to give him a precious spiritual lesson (2 Corinthians 12:7-10)" (Robertson and Plummer). Note periphrastic perfect active subjunctive of πειθω, to persuade.

In (επ), upon, both ourselves and God.