It was not Schlesser! Read the book.
Williams tried to trade Keke with another existing F1 team sometime after the 1982 Canadian Grand Prix. The driver concerned had yet to get much in the way of results by the time the book was finished at the end of '84. That gives a very limited pool of drivers as discussed earlier in this thread, with a shortlist of Cheever, Surer, Mansell, De Cesaris & possibly Jarier.
Schlesser was with an F2 team doing sod all in '82 and Senna was in FF2000. Non starter.
And as an aside, although Senna was very quick in his Williams test, there was never any consideration to take him in '84 as Laffite and Rosberg were signed up - the latter till end of '85 - with Frank & Patrick preferring experience.