And how do we know they're not going off of SM aces as opposed to USM? They're the exact same level in SM, last Pokemon be damned.
On the subject of Araquanid, there's also the fact that Ash himself has yet to see an Araquanid and that Araquanid has not been used for WTP yet. Both imply that that short little cameo in the Dewpider episode is not going to be the last we see of Araquanid. Of course, it is possible it just appears in a PotD filler or they just choose to use it for WTP because it happens to be in the background, and everything else has been used (kinda like how Alolan Ratatta is a likely candidate for the shrinking episode, because it's already debuted, Ash has already seen one, and it hasn't been used yet).
Also... I just don't buy that either Lana isn't going to get a single team change or Popplio evolving once is going to be all she gets. Something about both of those options seems very... incomplete. Not to mention, she'd be getting screwed over even compared to Mallow. No, I feel like either Popplio is going all the way or it'll evolve once and she'll get a second Pokemon. And if the former was going to happen, Popplio most likely would have evolved by now (unless it evolves in August-September, and it only remains a Brionne for around 20-30 episodes, which I feel is unlikely given how sparse Lana's episodes are).