It doesn't seem as though Russell was all that impressed with his first run in the Merc. 
D'you think he was given first drive considering LH is off to Ferrari next yr?

To be honest, no. Everything around this year's car, they need Lewis' experience and expertise. Difference between places in Constructors Chapionship is still millions. They might bring something more tomorrow. 
For next year's car, they will keep him out of the loop.

I think.

Absolutely. Who knows what the actual situation behind the scenes at Mercedes.