It's absurd by design for the sake of humor. The mechanics of some things may be slightly off from cannon. It was written just for the fun of CONSTANT VIGILANCE!
Alastor Moody always slept with his back to a wall, except when he didn't.
You can't always trust walls, even if shielded. There are always ways to get through. One wizard with knowledge of muggle heat sensing googles could find you and cast the killing curse straight through the wall as you slept. This was one of his own personal favorite assassination methods, but he did not need muggle devices to see people on the other side of a wall.
He had watched people's patterns enough to see how routine can bring them down. Hell, he had personally exploited the routines of Dark Wizards many times. He knew for a fact how effective that was, people are generally creatures of habit.
He had been awake a few minutes now, he remained laying down and pretending to sleep as the last of his wandless, wordless detection charms provided negative results. He then reached into his pocket, in a way that looked like a sleeping person shifting positions. He clicked the muggle pen he had in the pocket, it was an enchanted device to cast an anti-confundus on himself, in case he was confunded to think his test results were null. Then he repeated the previous spells, in a different order, a second time.
He did not uncover himself and get up as you would normally. He made himself visible now, an attacker would see that as him dropping his guard. With a pinch of will, a tactical roll and a pop he apparated around the corner of the hallway, which had been facing the bed. He was invisible once again. An attacker would be in a perfect ambush point. He would now be in the perfect point of ambush for an ambusher. Yet there was nobody there. Unless... He spun his eye around in his head. Nobody behind him or around, as he perceived.
His magic probed the tripwires in his wards, they were subtle. Although there was one tripwire specifically designed to activate when Moody himself probed it. He cast a reflexive shield to guard him from the backlash of colorless magic area effect. His wards were still active and untouched. He did not need to test them, but nobody needed to know that. Thanks to some lore Albus had passed down to him. His wards were impenetrable in the sense that he would know instantly if someone probed them. Unless someone had knowledge even Albus did not, which they probably did. All the same, redundancy was key.
He started to make his way to the kitchen, The Eye of Vance did not see anything amiss. Of course, he would not consume anything from the kitchen for breakfast, too easy to poision him that way. The enemy would likely also deduce this, and not bother to try. Still, he was following what would look like a predictable human pattern to a less sophisticated enemy. Wake up, go to kitchen.
Sometimes he pretended to go to the bathroom first. Though he cleared his bowels with a spell, sitting on the toilet was an opening an enemy may take, and he was prepared; the toilet would act almost like a Floo channel when activated and simply flush him out of the house and then self destruct with enough force to bring the house down.
Once in the kitchen, he rapidly recast a number of his own personally devised security charms. He often switched the order he cast them, sometimes he left some out, or added other new ones. This morning he cast a spell which would detect any lingering odors in the air which were not present the last time it was cast, such as body odor or magical creature secretions. Another spell would chime him if there were finger prints that did not match his own. Though it wouldn't be HARD to fabricate his fingerprints, but most wizards would not think to do such, except the ones that would...
After getting The Eye of Vance Moody had started looking into other magics which were related to the other senses. He did not need to cast the next one, he simply flipped a knut and it landed on heads. So he did cast his echolocation feedback charm, which as far as he knew could see through all methods of invisibility as long as the wizard was tangible. He had not run into any spells which made a wizard intangible, but if he could concieve it, someone else probably had already and devised such a spell. He flipped the knut again, heads. He wondered if he should STILL cast the other spell anyways, but decided not to, this time.
He was not as morally adverse to ritual magic as Albus was, and thus had no qualms with sacrificing the total sensory perception of one of the Carrow's cousins after catching him. It's not like the man had remained... Like that... For long. Moody had in that moment doubled the power of his own senses and his brain's ability to perceive these increased senses, it was almost an overload. If he wasn't already accustom to the extra information his brain took from his Magical Eye, the sesory explosion would have been overwhelming enough to drive a man mad. He of course had removed the eye before his ritual. The device was of ancient power already, unlikely to be affected, but it still may have interacted in some strange way.
His spell casting was done, and nothing seemed wrong. That's exactly when things can go wrong in the worst way. There was no point remaining in this disposable home for any longer. He stepped to the side and apparated to a new place. Not next door, but a few houses down.
He would watch the house he departed from for a half hour. He pulled out a mundane muggle binocular and spied the house for a half hour before he was satisfied nobody had found it, or at least that they were not going to move on it this morning.
Moody walked over to a wall, behind a false wall, and found the pin sticking out where he left it. He had enchanted it the last time he could be as certain as he could be that he was acting of his own free will. His magic told him that the pin was untouched. The pin was a button, and when pressed it cast a large number of "anti-spells" on himself, to rid any confusion or suggestion charms. This was redundant, but there was no harm in being redundant, as long as sometimes he neglected to be.
Once he was mildly satistified he was acting of his OWN free will, he walked over to the theif's downfall he set up and passed through.
He gritted his teeth and prepared himself for what came next. He pointed his wand at himself; "Cruico". It had taken work to self cast this spell. He wouldn't have thought it possible, until he received 'reliable' intel that Bellatrix could cast it on herself.
He had first preformed the curse on himself because he wasn't certain if criminals could avoid the pain in some way while pretending that they are in agony. Even occlumency was not enough to avoid the pain, as the spell worked on a body level more than a mind level, even though the mind is what perceived the bodies pain. There was some mechanic there he could take advantage of, yet he had not arrived at what seemed the right answer. He let the thought percolate in the back of his mind.
Albus was the only person he might have trusted to cast the curse on him. Of course the man would never, even if he could cast it, even to shore up security measures. Not to mention it would require Albus taking an unbreakable vow to not hold the curse for more than 5 seconds as well, incase somehow Albus had been hoodwinked. Then it would need hours of verification spells, running a blood test and other methods he would not even list in his own mind, to verify Dumbledore was Dumbledore.
So Alastor had to learn how to cast it on himself. This is how he learned that it is indeed possible to slowly build not a total block, but a 'resistance' to the curse for at least a few moments using all of his will power.
A possessing 'spirit' however, if there was one, would have to also experience the pain. (Unless the books lied, or there were books he had not studied, which there absolutely were). In that moment, no matter how deep it was in him, it SHOULD in theory flinch from the pain. For a powerful mage, it may only cause a microsecond of flinch, but that would be enough for Alastor to notice. Even if such a spirit could withstand it as he long as he could (up to 7 seconds), that flinch would be all he needed to activate the exorcism curse that was previously enchanted into his toe ring.
He could break his toe in a fraction of a second without raising his wand. Any ghost, spirit, soul or any other entity which had him hostage would be displaced. Unless they had forced him disabled the ring...
He had to call in almost ALL of his favors with the unspeakables to get access to some of their tomes on spirit magic. Once Albus had made him aware of the entire field, it had opened up a whole new set of vulnerabilities he was unwilling to knowingly possess. He even knew a thing or two about Horcrux's which he never told Albus, and had also used a reversible obliviate to rid himself of the knowledge. He had told Albus that much, which was probably a mistake but someone had to reverse it if the information ever was needed.
Oddly enough, while the torture curse was near blinding pain for those seconds he had grown to almost enjoy the after effects of a very brief torture curse. Having every muscle in your body clench so hard they almost tore from the tendons was a far better stretching routine than any he had used before.
He could now convince himself that he had a moment he compose himself and relax in the early morning drowsiness.
He leapt down to the floor and rolled in the opposite direction of his jump, and fired off a wide area stunner. When you relax is when they get you.
It seemed that so far this morning nobody was directly threatening him. Unless they wanted to build a sense of false security in him, ha! Not bloody likely. Yet still possible...
Imagine... If Moody himself was Voldemort, it's the last thing they'd expect. Which made it too obvious but also plausible.
Moody tore his own mind apart with legitimacy, searching for any crack to allow for intrusion. It was a pointless effort if Voldemort was in him, that one was far better at mind magic, and could hide himself surely. Moody simply hoped that the constant vigilance would make Voldy think it was too much effort to bother, if he ever made the attempt.
He cast his patronus charm and diagnosed it with a muggle brightness detector. If he was possessed, he could probably still cast it, but it would also likely dim by some small degree in the first moment of it's appearance. A half hour of spell casting later Moody was fairly sure he was not Voldemort, or that it was what Voldemort wanted him to think.