Fellow ADHDer here! (Hi friends!)
I just woke up so I'll try not fumble around with my words too much. I am gonna ramble a lot so uuuuuhhhh... Hang in there? ;v;
Syl's interlude was super relatable, even if it had ties in the supernatural. I don't think she's necessarily ADHD, being a little piece of Honor, HOWEVER she IS pure cognitive function, so... I don't see why not either lol. I think the personalities of a lot of radiant spren are meant to "offset" the personalities of their chosen radiant, see: Kaladin gets-his-head-stuck-in-the-crem Stormblessed, tempered with Syl Born-to-be-free Phrena. Or Teft who gets trapped in self-loathing spirals, and Phendorana, kind-and-patient-but-firm, who'll call him out on his non-sense. So... Syl being distracteable maybe in direct foil to Kaladin who has a hard time LETTING himself be "distracted".
As for Rushu I'm... Hesitant. She's got the inattentiveness, only maaayyybe hints of executive dysfunction, but I haven't seen much evidence of rejection sensitivity dysphoria.
Like in Dawnshard,
If that were me in that situation, I'd be getting inundated with shame spren whether I like it or not. (If it hadn't been polite, it probably would've been anger spren. Reactiveness is funny (annoying) like that.)
RSD is shown in, and I think I'm remembering this right, 97% of ADHD folks. Rejection, be it litteral or even PERCEIVED, feels like a huge mental punch in the gut, even over little things. It can be confused for bipolar disorder if it's really bad, but can spiral off into depression, avoidance disorder, social anxiety, etc. Depending if the ADHDer in question is prone to externalizing or internalizing. That sensitivity, in combination with the other a Big Two (Three if you include hyperactivity *points at self*) can result in, well... A Very Annoying Character. Seeing how polarizing (whomp whomp) Shallan can be in the fandom (I've avoided those forums bc I LOVE Shallan, as an artist and as someone who copes/coped with trauma by... Blanking.) I'm anxious about whether I actually want to see her shown with the full ADHD buffet.
THAT SAID... Navani is just about the best patron/master/guide one could ask for. She never blames or berates Rushu for Being Rushu, just gently and firmly guides her on her own path. Which makes for an engaging yet low-stress environment for her to flourish! (I'm a little envious, actually.)