We will never know fro sure, but I really had the feeling the Hulk was called in to allow Stroll to get in front of him. Father Lawrence would never allow his son to become second behind a guy who just did his first race in the Mercedes replica. And why did they put him on the red tyre, which everybody else seemed to avoid. Personally I also thought the Hulk was a bit short in his interview with Dutch television. Which gave me the impression he was a bit annoyed.
So Lance Stroll will definitely keep his seat, as long as his father has the money to pay. Vettel currently seems the man with the best cards to take the second seat in the team (at least in my opinion).
The Hulk is a very experienced driver so I think he will have the best chance to get back in a seat of a team which needs some one who knows how to set up a car.
Cheers,
Huub