This man might have been set at liberty (Apolelusthai edunato ho anthrōpos houtos). Conclusion of the second class condition (determined as unfulfilled) without an as in Act 24:19 because of edunato (verb of possibility, Robertson, Grammar, p. 1014). Note perfect passive infinitive apolelusthai from apoluō. He certainly “could have been set free.” Why was it not done?
If he had not appealed unto Caesar (ei mē epekeklēto Kaisara). Condition of the second class with the past perfect middle indicative (op. cit., p. 1015) of epikaleō (cf. Act 25:11.). But Paul only appealed to Caesar after Festus had tried to shift him back to Jerusalem and had refused to set him free in Caesarea. Festus comes out with no honour in the case. Since Agrippa was a favourite at court perhaps Festus would be willing to write favourably to Caesar.