I think myself happy (hēgēmai emauton makarion). See note on Mat 5:3 for makarios. Blass notes that Paul, like Tertullus, begins with captatio benevolentiae, but absque adulatione. He says only what he can truthfully speak. For hēgēmai see note on Phi 3:7 and 1Ti 6:1 (perfect middle indicative of hēgeomai), I have considered.
That I am to make my defence (mellōn apologeisthai). Literally, “being about to make my defence.”
Whereof I am accused (hōn egkaloumai). Genitive with egkaloumai as in Act 19:40 or by attraction from accusative of relative (ha) to case of antecedent (pantōn).