Interaction of Intellect Devourer's actions with the Protection from Evil and Good

Question

How do Intellect Devourer's (ID) actions interact with the spell Protection from Evil and Good? Am I right that:

1. if protection from evil and good is active on a creature before the ID could successfully use Body Thief on it:
   1. The "Claws" attack has disadvantage on attack rolls against the creature,
   2. if the ID uses "Devour Intellect" does the target have an advantage on Intelligence saving throw (looks like it's not stated directly in the spell's description, there is just a note about this if the target is already "charmed, frightened, or possessed") when the ID uses this attack for the first time? Or the next time, if it previously failed?
   3. The ID cannot use "Body Thief" at all on the protected creature.
2. if "Body Thief" has already been used successfully and the target creature was not under the effect of the spell (which is a consequence of the previous statement, otherwise it cannot be), then:
   1. The ID's attacks (on the first target) have no meaning because the target's brain is magically consumed and ID is in the target's skull,
   2. casting "Protection from Evil and Good" on the target with the ID inside will with a chance of 100% drive the ID out.

Am I right? Please, pay attention to the 1.2 statement.

Actually, this question is not a duplicate of this: Protection from Evil and Good and Intellect Devourer because there was asked only if the Protection from Evil and Good can drive the intellect devourer out of the body as is stated in the monster description while in the spell description it is stated that it can affect only the creature (not body).

Answer 1

Body Thief cannot be used on a protected creature.

The targeting rules of Body Thief state:

[...] that isn't protected by protection from evil and good.

The intellect devourer has disadvantage on attack rolls.

The intellect devourer is an aberration and therefore has disadvantage against creatures that are protected by protection from evil and good. It is also true that attacks of the intellect devourer against a creature are meaningless once Body Thief was successfully used, although this cannot happen while the spell is in effect (vide infra).

Body here means creature without brain, not dead body.

The rules for driving out the Intellect Devourer state:

If the host body dies, the intellect devourer must leave it. A protection from evil and good spell cast on the body drives the intellect devourer out.

If the body is dead, the intellect devourer must leave it anyway. The part about the spell therefore applies to the brainless but living body.

The spell only grants advantage on saving throws against being charmed, frightened and possessed.

Protection from evil and good states (emphasis added):

If the target is already charmed, frightened, or possessed by such a creature, the target has advantage on any new saving throw against the relevant effect.

This is not applicable for devour intellect because a) it includes none of the named conditions and b) it does not apply any ongoing effect calling for a saving throw later. Both of these conditions need to be met.

A trait where the spell is applicable is for example the Frightful Presence of dragons (here from an ancient white dragon):

Each creature of the dragon's choice that is within 120 feet of the Dragon and aware of it must succeed on a DC 16 Wisdom saving throw or become Frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.