I think ZGuitierez will stay. He is not doing a bad job.
As for Button being a 3rd driver. He actually has the best deal of the 3 drivers. He probably knows mclaren wont be at the front, and i think he saves himself that frustration. He can also find time to do other things and develop himself outside of Formula1.
And with a 2 year deal where can drive any race if the need arises, what's not to like?
I actually think his deal is pretty good. He wont be winning the WDC next year, so might as well enjoy the ride developing the car, racing the odd race, and then still being available for 2018 should stoff end up being rubbish.
Alonso is the one who should be worried about his future. He has to face another rookie teammate. Lets see if he throws the toys out the pram again.