The Ghost is unaffected by the Confusion spell. While there is no strong statement that all creatures affected by a spell are "targets" of the spell, I did find this on page 205 of the PHB:

Many spells specify that a target can make a saving throw to avoid some or all of a spell's effects. The spell specifies the ability that the target use for the save and what happens on a success or failure.

This seems to imply that the targets of a spell are any creatures affected by it, although it's obviously not as clear as we could hope. Another example, on page 196:

If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them. For example, when a wizard casts fireball or a cleric casts flame strike, the spell's damage is rolled once for all creatures caught in the blast.

As backup for this, we have the Ghost's Possession ability on page 147 of the Monster Manual:

The ghost can't be targeted by any attack, spell, or other effect, except ones that turn undead[...].

Turn Undead is an AOE ability, not a targeted one. If the Ghost was still meant to be affected by AOE spells and abilities, the caveat for Turn Undead would not be necessary.

And finally, if AOE spells did affect a Ghost in the midst of possessing someone, the Ghost would be affected by every Fireball or Lightning Bolt that hit the possessed creature. In your fourth point, you asserted that this obviously wasn't the case, which I agree with. There's no reason why Confusion would work differently than every other AOE spell.

... the creature gains an intuitive understanding of how to activate any magical properties of the item, including any necessary command words.

The ghost has not done this and so lacks the "intuitive understanding". The brain/soul/mind/whatever inhabiting the body is not the one that did the attunement.

Without becoming attuned to an item that requires attunement, a creature gains only its non magical benefits, unless its description states otherwise.

The ghost is not attuned to the item so it functions as a non-magical item of that type.

The sun blade (aka light sabre) would vanish leaving only the hilt. The ghost could use this as an improvised weapon with no magical bonuses if it wants.