[Antilochos]

First question to me is yes as well; proceed with Ocon education in 2020 with a race seat (if he is showing good data of his speed in simulator).
Long term Ferrari have Leclerc I'm sure, so Mercedes has to get their next "Hamilton" ready. But, like you also said Jody, there is Verstappen and if Red Bull doesn't get their car competitive, big change Verstappen walks towards Mercedes. Don't see Ocon along Verstappen and Mercedes might take Verstappen over Ocon (I would; better choice probably in racing, in developing car and as a bonus also for marketing). So, bet on Ocon but if Verstappen comes available, go for Verstappen.

In that perspective, we go to the next question; should Ocon take Wiliams seat? Yes. It is a shit car, but it puts him on the radar for the whole F1 world a bit more. He might need that for if Verstappen would join Mercedes. The data he will provide with the Wiliams will still give Mercedes all the info they need to judge Ocon and he stays available for Mercedes if needed.
Risk for Ocon is Norris, but if he believes he is worth driving for Mercedes, for a championship, that risk of Norris would be no issue.