Present participle and so naturally meaning that the heaven was opened while Jesus was praying though not necessarily in answer to his prayer. First aorist passive infinitive with double augment, whereas the infinitive is not supposed to have any augment. Same construction as the preceding infinitive. The Holy Spirit. This probably means that the Baptist saw the vision that looked like a dove.
Nothing is gained by denying the fact or possibility of the vision that looked like a dove.
God manifests his power as he will. The symbolism of the dove for the Holy Spirit is intelligible. We are not to understand that this was the beginning of the Incarnation of Christ as the Cerinthian Gnostics held. But this fresh influx of the Holy Spirit may have deepened the Messianic consciousness of Jesus and certainly revealed him to the Baptist as God's Son. Same construction of infinitive with accusative of general reference.