When do heavenly hosts tremble if not here during the Gen 6 incursion referenced in 1 Enoch 13?
One such place would be the liturgies of John Chrysostom and Basil the Great, which read:
No one bound by the desires and pleasures of the flesh is worthy to approach you or draw near to you or minister to you, King of glory; for to serve you is great and fearful (φοβερόν) even for the heavenly Powers.
As you are probably aware, ancient Christian liturgies have historically evolved from earlier Jewish liturgical services. This idea in particular seems based on passages such as Isaiah 6:2-3, wherein the seraphim themselves are forced to cover their feet and faces with their wings, so as to protect themselves (as Moses earlier in Exodus 33:18-23) from God's unfathomable glory. Job 15:15 also appears to express a somewhat similar idea, as does the penitential prayer of king Manasseh:
O Lord God Almighty, [...] you who have made heaven and earth with all their order; who have shackled the sea by your word of command, who have confined the deep and sealed it with your terrible (φοβερῷ) and glorious name; at whom all [things] shudder (φρίττει), and tremble (τρέμει) before your power, for your glorious splendor cannot be borne (ἄστεκτος) [...]
If James is not referencing 1Enoch 13:5 then from where does he get his information about δαιμονια daimonions, that they believe in God yet tremble/shudder?
One possible source would be Jewish exorcisms, some of which were performed by Christ himself:
Mark 1:23-24 ¶And there was in their synagogue a man with an unclean spirit; and he cried out, saying: Let us alone; what have we to do with thee, thou Jesus of Nazareth? art thou come to destroy us? I know thee who thou art, the Holy One of God.
Mark 5:2-7 And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit, but when he saw Jesus afar off, he ran and worshipped him, and cried with a loud voice, and said: What have I to do with thee, Jesus, thou Son of the most high God? I adjure thee by God, that thou torment me not.
Luke 4:33-34 And in the synagogue there was a man, which had a spirit of an unclean devil, and cried out with a loud voice, saying: Let us alone; what have we to do with thee, thou Jesus of Nazareth? art thou come to destroy us? I know thee who thou art; the Holy One of God.
Luke 8:27-28 And when he went forth to land, there met him out of the city a certain man, which had devils long time, and ware no clothes, neither abode in any house, but in the tombs. When he saw Jesus, he cried out, and fell down before him, and with a loud voice said: What have I to do with thee, Jesus, thou Son of God most high? I beseech thee, torment me not.
As can be easily glimpsed from this pair of parallel passages, the fearful demoniacs profess knowledge of both God and His Son Jesus Christ.
Is James 2:19 referencing 1 Enoch 13:5?
Possibly, but inconclusive.