I just had kind of a crazy idea:
What if we use the audio from that deleted scene of Finn confronting Phasma and had him say it earlier - like, when he’s about to be executed? Then the stormtrooper turns around and decapitates Phasma (this wouldn’t need to be shown in too much detail, since the scene is intercut with a lot of other stuff. Maybe just add an animation of the lightning-axe-thing swinging over one of the shots of Phasma turning her head). Then, remove the fight and have Finn and Rose go straight to the shuttle - but while this is happening, repurpose some of the shots from later of stormtroopers running away to make it look like they’re escaping too.
And this is the tricky part - change the conversation between Finn and Jannah in TROS to make it so that she was the executioner.
You could even take it a step further and reinstate the deleted scene of Finn convincing a Stormtrooper to let him go, but edit a black “executioner” stripe onto his helmet, and dub him with a female voice to that it’s also Jannah.