I'm completely out of ideas of how to deal with my current situation.

My mummy's inventory shows my other sims cop car in it. Purging his inventory with NRaas debugger removes both his non interactable stuck version and the other workable version be it in the other sims inventory or on the lot.

Wondering if anyone has even encountered this before?

Would my best route be sucking it up and cheating using the mail box to recover a new car? I tried using it before but my sims career just would not change.

I'm not sure I understand; you mean that there's one sim in your family using another member of the family's car? Because that's normal behavior: the car is for the whole household and it's a bit random who will take it. i.e. there's no ownership like in Sims 2 where each member of a family could have his own vehicle and always sticks to it. You can set a car as 'favorite vehicle' of sim A but that won't stop sim B from your family from taking it to go to the grocery shop, if the car is available because it's set on a parking space and therefore it ends in the inventory of sim B. Just drag the car from the inventory to your parking space to put it back for sim A to use. If you sold the parking lot the car will be stuck in the inventory but the sim carrying it will still be able to autonomously use it. If you want only sim A to use it, drag it to his inventory then sell the parking space, same thing for a bike or a motorcycle BTW.

If this helps, to use the mailbox to obtain a new job, you'll need to quit your current job first.

Well switching careers then going back to Law Enforcement doesn't work like a youtube video showed off. Maybe its one and done per the sim or something.

@MiniMimi it isn't simply the sims borrowing each others car per say rather a duplicate image in the other sims inventory that I cannot interact with. It is also tied to the original and deleting said original removes the stuck image from the other sims inventory.

I'm guessing it all started from the mummy stealing his vehicle outside of work after grabbing his award down at Townhall.

Eureka I fix it! Placed a parking spot at the house and took the functioning cop car and placed it down in the spot. Then had the mummy go down to the store. Had some heart palpitations as after he arrived no cop car in parking spot at house BUT his inventory was cleaned of the stuck image. Drove back home IN the cop car and now all is well in the household. Going to be giving both the Genie and the Mummy their OWN vehicles and pray this doesn't happen again!

Ah, you didn't have a parking space, so that's why the car was stuck in your sim's inventory. Hmm I thought the car would still be autonomously used by the sim carrying it in that case. You can see how rarely I buy them any cars lol I generally use bikes and solex/vespas and love putting those small parking racks everywhere.
Anyway I'm glad you solved it and retrieved your car! Yes, the cop car is awesome: when your sim is using it you can click on the car and select "turn on the siren" or whatever it says in English :D

Cars & motorcycles are proving to be a headache in my game. NRaas OR another mod likes to nom them on load up so I make sure to save when no one is driving. I think its a scripting issue. Parked ones are safe but the driven cars are not.

I don't have a parking lot on the home lot as there really isn't any room, also because it would mess up the horse's already poor routing on the lot, unless I carved a basement to shove em all in which would be an eyesore.

Completely forgot about the siren option, thanks!

protip: you can place the parking lot anywhere with moveobjects on, like in the attic.. And yeah it should still work.
Haha the siren also works for the firetruck but too bad you can't buy it ^^

Haha the "attic" of the castle is already in use by the resident alien! I also have the HUGE science machine thing that I really don't have anywhere to place. I may just buy another lot to place it on. (Maybe the university?) I don't have much experience with how real estate works.

Going to get that firetruck eventually too! Just have to wait till the time is right and the sim I have planned to become a firefighter joins the household. Not sure the exact way I want them to join yet. Maybe come next Spooky day!
(Eventually 19 sims, counting pets, in the household planned...)

Wow, you're indeed going to be crowded! :D
How about a secret basement? You don't even need any access to it if it's only to put your parking lot(s) in it! And from what I'm reading you can have a max of 4 levels of basement, that's plenty of place to stash all your stuff, you may even make it usable if you manage to put a stair somewhere (that the hardest part), then you'll be able to put all your machinery in it (you could build 2 open sub floors to put a huge machine, like a duplex). I've just quickly launched the game to test that, put a parking space in a totally inaccessible basement (sub level 2!) and no problem for the car, the sim used it and returned it alright. If you need to put an access, IIRC the fire pole works perfectly in basement and it takes a bit less space than normal stairs (and looks nifty, like a secret agent lair), there's also spiral stairs and elevators. Make a huge secret alien lab in the basement and you'll be able to park your cars in the attic!