Honda pays Alonso, and has a say in that. Alonso won't go. If Alonso goes, Honda has word in who replaces him.
As for VanDoorne, Honda has nothing to say in that. That is purely Mclaren and Mclaren decide to use one of their development programme protegé's because he'll be loyal,
he's known to them and gives them easy information that they can use, there is no pressure because the car is far from fast either way, and he'll be cheap coming from their own
programme and additionaly thus very manageable.
If Honda brings fast engines and they'll be up for wins, hopefully next year, then VanDoorne needs to deliver OR he'll get replaced fast. Hence why Button was still signed. If Honda's engine performed, and VanDoorne wasn't up to the task, he'd be replaced by Button who has proven himself, and the prospect of winning would have given Button reason to compete. That chapter however is closed now and it's simple as that.
If Vandoorne doesn't deliver next year when there's a fast car, he's out and Mclaren will have plenty to choose from - as long as the engine is good offcourse. Which would include Hamilton, Perez, Ocon [though Mercedes probably wont want to ditch him], Verstappen, Ricciardo, Bottas, Vettel, Raikkonen, etc. etc. All already matured and shown to be capable. Something VanDoorne has not shown in any way yet.
Dumpster sounds so much more classy. It's the diamond of the cesspools.