Is there a way to become immune to Possession all the time?

I’m going to be running an Eberron game soon, and one of the players has expressed an interest in playing one of the Inspired ‘gone rogue’. Unfortunately, the rules for Possession of the Inspired are … Very much in favor of the Quori.

I can always come up with a custom item, but is there a way to become immune to possession more or less all the time in 3.5? Answers that work from moderate levels are preferred, but anything that works will give me something to work with.

I am looking for pre-existing solutions which are rules legal as a starting point.

Can the Nystul’s Magic Aura spell make you immune to/unable to be targeted by the Dominate Person spell?

Nystul’s Magic Aura can make it so that magic and spells treat a creature as though it were another creature type. However the extent of this effect is ambiguous.

At first, it says the following, which specifically mentions spells which “detect creature types”. One could argue that this means it only affects information gathering spells.

You change the way the target appears to spells and magical effects that detect creature types, such as a paladin’s Divine Sense or the trigger of a symbol spell.

Then it goes on to give a far more general rule:

You choose a creature type and other spells and magical effects treat the target as if it were a creature of that type or of that alignment.

One could use this to change one’s effective creature type to something like Dragon or Celestial. This would seem to make you immune to spells that require a humanoid target, such as Dominate Person, as well as magical monster abilities which require a humanoid target, such as the Vampire’s Charm ability.

One could argue that Nystul’s Magic Aura is intended to only fool spells which gather information, but one could also argue that any spell which only works on a specific creature type is one which gathers information, even if that is not its primary purpose.

Which is right? Would Nystul’s Magic Aura make you immune to Dominate Person?

Does immunity to piercing damage make you immune to the pull effect of the spell Thorn Whip?

If a character has gained immunity to all piercing damage, would that character also be immune to the pull effect of Thorn Whip?

For reference:

Thorn Whip
Transmutation cantrip

Casting Time: 1 action
Range: 30 feet
Components: V, S, M (the stem of a plant with thorns)
Duration: Instantaneous

You create a long, vine-like whip covered in thorns that lashes out at your command toward a creature in range. Make a melee spell attack against the target. If the attack hits, the creature takes 1d6 piercing damage, and if the creature is Large or smaller, you pull the creature up to 10 feet closer to you. This spell’s damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).

Are creatures that are immune to disease affected by the harm spell?

The harm spell, flavourfully, states it creates a virulent disease

You unleash a virulent disease on a creature that you can see within range.

It later calls out that

Any effect that removes a disease allows a creature’s hit point maximum to return to normal before that time passes.

But no where does it state that being immune to disease (via the Paladin’s Divine Health class feature or otherwise) prevents either the initial damage or the reduction in maximum hit points.

This is in contrast to spells that spell out if immunity works, e.g. sleep.

Does immunity to disease protect you from the harm spell?

Are undead and creatures immune to charm excluded from the HP count of Sleep?

From the spell description:

Sleep (…) Creatures within 20 feet of a point you choose within range are affected in ascending order of their current hit points (ignoring unconscious creatures). Starting with the creature that has the lowest current hit points,(…) Subtract each creature’s hit points from the total before moving on to the creature with the next lowest hit points. A creature’s hit points must be equal to or less than the remaining total for that creature to be affected. Undead and creatures immune to being charmed aren’t affected by this spell. (PH p.276)

Now, to build the ascending order of their current hitpoints, the spell instructs to count everyone in area of effect, but only ignore unconscous creatures. Then later on it says that Undead and creatures immune to being charmed aren’t affected.

Are those two latter kinds of creatures excluded from the roll of current HP by default? Following the “if they were it would say so” philosophy of Sage Advice, it doesn’t seem so to me.

Do creatures immune to poison still make saving throws?

At 11th level, monks gain immunity to poison. The Poison Healer feat lets you restore hit points when you “succeed on a Fortitude save against a poison”. Can monks chug poison to heal themselves, or do they not actually make the saving throw?

(There seem to be similar questions for other editions, but I can’t find one specifically for 3.5e)

Does an Iron Flask (5e) work on a creature that is immune to Charm?

The Iron Flask states, “You can use an action to remove the flask’s stopper and release the creature the flask contains. The creature is friendly to you and your companions for 1 hour and obeys your commands for that Duration. If you give no commands or give it a Command that is likely to result in its death, it defends itself but otherwise takes no Actions. At the end of the Duration, the creature acts in accordance with its normal disposition and Alignment.”

Now my issue with this comes from the following incident that just occurred in my past session for Baldur’s Gate: Descent into Avernus. The players traversed the Scab prior to entering the Bleeding Citadel to find the Iron Flask amongst the deceased Night Hag’s obejects in her lair. They released the Fiendish flesh golem and killed it due to it not being their’s and attacked them so says the module. They still held onto it after the wizard identified it and realized its potential for demons if anything. So they eventually came across the diggers trying to dig out the Crokek’toeck. They killed them and reached the beast themselves to capture it in the Iron Flask instead. The bit I’m concerned with is they intend to use the creature as a trump card and have it serve them in battle for the hour use.

What I don’t understand is that the item doesn’t explicitly state that this is a charm on the creature to force its friendly position towards the party and adhere to their commands. The crokek’toeck has immunity to charm and confuses me as I presume the Iron Flask technically charms said captured entities to obey their temporary master(s). Due to the fact the item neglects to state this; do I proceed with the crokek’toeck being immune to the friendly nature once it pops out and proceeds to flee, attack, etc the party, or is this Flask meant to override that immunity and it works as they think it would, therefore obeying the party? I’m seeking a logical answer prior to flavored suggestions.

Can a Paladin use Divine Smite when attacking (with a natural 20) a creature immune to the weapon’s damage type?

Prompted by this question about Sneak Attack and Immunities, it seems a character can hit (although for 0 damage) with a weapon which deals damage to which the target is immune.

What would be the result if a Paladin rolled a 20 (with a non-magical slashing weapon), attacking a creature immune to non-magical slashing damage, if the Paladin casts Divine Smite?

Note: I specify a natural 20 in the actual question to avoid discussion about armor class when considering the result.