Not sure if it helps get more information, and this actually has no competitive impact at all, but I noticed something about Fishious Rend and Beak Bolt.
My Scarf Dracovish was sweeping Hop's final team in the post-game by Fishious Rending everything in sight until it failed to OHKO his Snorlax.
Hop used a Full Restore... and Fishious Rend dealt less damage than before (the first hit put Snorlax in red health; the second hit put him on yellow health with a critical). So there are more actions that prevent double damage.
This makes me think there is just one condition for Fishious Rend and Beak Bolt to deal double damage: the target Pokémon must have not already used its current turn. Things like attacking, using a status move, or the trainer using an item count as using a turn on the Pokémon, whereas a Pokémon that has just switched in has not used a turn yet.
So the question changes to "Does not being able to move (e.g. flinching) count as using your turn?". I guess it does, but it's best to check...