I don't know I still wouldn't say that Ash catching a Salandit wouldn't really be redundant considering a potential Incinroar for Ash will probably have 2 Fire type moves, 1 Dark type move, and 1 Fighting type move while Salazzle would probably have 1 Fire type move, 1 Dragon type move, and then either 1 Poison type move and 1 move of another type or 2 Poison type moves, so I don't think that a shallow move pool for Salazzle really matters as there is enough offensive moves that it can get out of the move set to make it work.
As for the whole speed not matter to Ash as much as people think it does while mobility does matter, you make it sound like you somehow think Incineroar is somehow more mobile then Salazzle or that Salazzle isn't that mobile, the thing is that given Salazzle's body shaped, speed, and weight I can't see how Salazzle wouldn't be that mobile, if anything I think it would be more mobile then Incineroar.
That's still not adding much to Ash's team other than being a Pokemon that can use Poison Type moves and being speedier than Incineroar. Did Leavanny really add much to Ash's team when he already had Snivy? Did Palpitoad add to Ash's team when he already had Oshawott? No. Not really. Hell, even Kingler didn't do anything unril the league.
I only brought up the speed thing in case somebody wanted to counter what I said about Salazzle not adding anything to Ash's team by saying that it adds a fast Pokemon to his team. All speed does is make battles flashier, and people only think it matters because he had all speed demons except for Goodra in XY(Z). That wasn't true in previous sagas, where it was more 50/50. As for Salazzle being more mobile than Incineroar, I don't know. I could imagine Salazzle being able to dodge attacks easier because it's faster and a smaller target, but that's about it. Incineroar may be slower, but it's not an immobile tank like Torterra or Torkoal or Boldore. It is probably still capable of dodging attacks just fine, plus it can punch, kick, bite, claw, etc. where Salazzle can't.
As for Plumeria, if she's the one stopping Ash from getting Salazzle, it's as simple as changing her ace to Toxapex or even an older Pokemon. You can change Plumeria's Pokemon, but you can't change their type, battle styke, or movepool.
I don't know. I just feel any two of the four Pokemon everyone is speculating Ash to get (Araquanid, Golisopod, Kommo-o, Alolan Sandslash) would add a lot more to Ash's team than Salazzle.