I do hope Mercedes are smart enough to put Wehrlein in the seat on a race by race basis. It affords them the flexibility to test him out while leaving options open should he not meet the required performance level - though I have little doubt he wouldn't.
Beyond that I distinctly remember the Hamilton situation where he was ready for the McLaren seat a year, maybe even 2, before he was given it simply due to McLaren being far too conservative.
Can't see it happening. They will want to have made a final choice by the start of the year, its no good trying to bring someone in after 4 races if he's not up to scratch as it'll be even hardy to pry someone away. Plus they'll not know the car at all.
Get it done fast so they can sit in the simulator for 6 weeks.