If Oak has the 19 Meltan at the lab, I hope that what could happen is that he researches them a bit longer while Ash his own Meltan. Then before the Alola League starts, Oak decides to send them back to Alola and assuming Meltan evolves by gathering together, something will happen that requires all 20 Meltan to evolve into Melmetal which Ash then can use for the league.
And then once Ash is done with the league, Melmetal can be Oak'd once Ash gets back to Pallet so he can research Meltan's newly evolved form. That's probably a scenario I can see that can have Ash keep his Meltan/potential Melmetal.
And then once Ash is done with the league, Melmetal can be Oak'd once Ash gets back to Pallet so he can research Meltan's newly evolved form. That's probably a scenario I can see that can have Ash keep his Meltan/potential Melmetal.