Russell to swap for the weekend is possible but what's the real upside ? - ignore the bs being written about improving the standing of Williams - Bottas is locked in at Mercedes and Russell is locked in at Williams. Mercedes would have all the information they need on his potential.
You can never have enough information.
Another upside is they get to try out their next in line for a seat as Toto calls him.
Williams gets money from Mercedes for Russell and money from whatever pay driver they take on.