The crucial point here is that these two sentences, in which the FP is placed inside the embedded clause, cannot, in any of their interpretations, be equivalent to (6c), in which the FP is in the matrix clause.
if the FP in (6a) were to associate with the matrix object NP which contains the whole embedded clause inside itself as a relative clause, the interpretation of this sentence would turn out to be equivalent to one of the several possible interpretations for (6c) in which the FP associates with the NP it attaches to.