How do protective enchantments work on equippable non-armor items (elytra, mob/player heads)?

As SpiceWeasel said in the comments, landing with an Elytra deals no damage (if you land correctly without hitting anything)

I believe this is because fire protection is random at choosing whether or not the armour absorbs the damage (once per tick), and if the probability of absorbing the damage is 0.5, then that would explain the 2 armour pieces. I believe this because I saw this video (from 5:30~) of Xisumavoid swimming in lava and sometimes it dealt damage, other times not.

To reduce fall damage, I think you can use the resistance effect using /effect give @p resistance 100 255 true