Regarding Jannah, I think it would be a bit of a pacing issue to introduce a seemingly random stormtrooper in the midst of the story in a way that could feel organic. Maybe a throwaway reference could work (a name drop for “TZ-1719” over comms).
You’re the guy who suggested readding the elevator scene with the troopers and Finn, where the trooper congradulates Finn. You even proposed having that same trooper be the first one to turn on Phasma if JJB adds “Phasma’s End” into the novel. The trooper could simply be changed to female, and we get to meet Jannah in TROS as usual but ties to this hypothetical whatever.