Latest speculation on the Mercedes second seat is the decision has been made, but they are delaying to announce where Bottas is going due to request by that team - who wants to announce their driver lineup in September.

The racing team in which the losing driver will be placed would like to announce their team for 2022 in September.

Thus, the decision for the second driver may have already been made by Mercedes, but the timing of the announcement depends on, for example, Williams or Alfa Romeo, to whom Bottas is linked, among others. The drivers themselves seem to know more, but they will have to keep their mouths shut for the time being.
https://www.gpblog.com/en/news/92165...nd-driver.html