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Like, Anakin might not know that Rex defected. He promoted Appo but technically Appo’s still second in command of the 501st, under Rex (Rex is still listed as a Captain because Anakin is really really fucking awful at paperwork). And then when Rex doesn’t show up, the obvious conclusion is that he’s dead, but Anakin still doesn’t let him get listed as MIA or KIA because that’s HIS CAPTAIN, dammit, and he’s lost Ahsoka, Padme, Obi-Wan, and literally everything else he ever cared about, he doesn’t want to face having lost Rex, too. And he works better with Rex than with anyone else (mostly because Rex has become accustomed to his bullshit and is Very Good at taking Anakin’s spectacularly stupid nonsense and Making It Work), and really he can’t picture leading the 501st with anyone OTHER than Rex at his side.
And then Rex keeps not coming back. But Vader still doesn’t remove him from the system. For years and years. And eventually everyone stops mentioning it to him because they tend to get force-murdered if they do.
So long story short, Rebels-era Rex is technically the commanding officer of Vader’s Fist, and the system still recognizes his codes and lets him in, and technically if he walked up to any stormtrooper while wearing his old helmet, the networks would recognize him and tell the stormtrooper that’s your CO. So if Rex realized this, he could just waltz in and take control of the most feared stormtrooper legion in the galaxy and as long as Vader wasn’t around, no one would stop him (they’d all be very confused, but they wouldn’t stop him). And maybe vader walks up at this point like “what the hell is going on here” and sees Rex and is just like “REX there you are where the FUCK have you been?” and Rex is like “uuuhhhh i had engine trouble on the way back from Mandalore” and it’s been 20 years but Vader knows how shitty Rex is at piloting and really anything involving flying and ships so he’s like “sounds legit”