I think everyone is taking this too literally. Although M-Sport has said two events that isn't necessarily so.
Petter may stick with Jari's setup to start with (lets face it, he's had a lot of seat time in the car so it's a good starting point) & make only minimal changes. This should allow him time to settle into the car under the heat of competition & learn to push it to it's max. Only once Petter has pushed the car to it's max will he learn the downfalls of the car & then he will be allowed to start suggesting & making radical changes. If however Petter is able to find the limit of the car during the first or second event I am sure the engineers will listen & allow changes.