Lucifer Morningstar

The exceptions being when God Johnson uses it, which, (if I recall rightly) after being corrected, doesn't use it again. The other time being when Amenadiel manipulates Linda into emotionally harming Lucifer by deadnaming calling him Samael. To which Lucifer had, to date, the most violent reaction to anything Linda has said in her sessions.

So you're telling me that in Lucifer Season 4 Episode 9:

Lucifer and Chloe worked on a case that involved twins, where one was suspected of killing the other

Lucifer was going through an "i hate myself" crisis and we got this line

AND MICHAEL WAS NAME DROPPED

Calling Trixie “Trixie” instead of spawn/urchin/Beatrice/etc. 

Calling Ella “Ella” instead of Ms. Lopez 

Being nice to Dan lmao and not calling him Detective Douche 

Maze just straight up telling everyone whenever her fight with Michael happens 

He probably knows basics (ie how Lucifer dresses, the accent, probably the whole desire shtick, the fact that he and Chloe are in love) but maybe not main events - like Charlie, for example, or being vulnerable in front of Chloe? 

Michael puts on gloves before he handles any evidence

He actually questions witnesses instead of just accusing them of a crime

He doesn’t try to connect every case with whatever he’s been angsting about that day

He treats Ella like a colleague/nuisance instead of like a little sister who he quietly adores (Azrael and Michael hate each other no I will not be taking criticism my goth bby has standards)

He doesn’t sing/play the piano at Lux or if he does he does it badly and only sings Top 40 hits

He actually listens to Dr. Linda the first time she suggests something

He is worried about making God angry or simply seems unwilling to risk His wrath (let’s be real, Luci is frothing at the mouth to piss off his dad)

On a related note, Michael is probably in the Remiel camp of “half-angel baby = bad” while Lucifer honestly gives zero fucks so that might be a sign

He follows any and all police procedures without Chloe having to twist his arm. Dead give away.

That's certainly an interesting theory! The dark Deckerstar counterpart is a rabbit hole I'd actually be okay with falling down. It would really emphasize those discrepancies between Lucifer and Michael, especially if Michael seemed invested in the relationship. However with that being said, I'm not sure if Lilith will ever come to fruition in this universe. I'd definitely be interested in seeing her, but I'm not sure if there will even be time to introduce her, what with the audience still adjusting to Michael. But anything can happen and I've certainly been wrong before so I wouldn't discount your theory just yet ;)