It turned out that https://issues.redhat.com/browse/PLANNER-2599 is not doable. In other words, problem changes cannot be fully incremental.
Thus, it makes no sense to require users to call methods that were meant to support incremental changes: addEntity, removeEntity, addProblemFact, etc.

Instead, the ProblemChange implementation is supposed to directly change the best solution passed to it.

Every ProblemChange should also decide whether or not it expects shadow variables to be already updated on the best solution it receives:

default boolean usesShadowVariables()