Of course, given that feature percolation is possible only from the Spec (and the Head), in order for this feature percolation and the subsequent pied-piping movement to be possible, the WH-phrase such as kau d 8 must move 'covertly' to the Spec of the pied-piped phrase.
as for the Complex NP island violation cases such as (24a), I will assume, following Nichigauchi (1992) and Ortiz de Urbina (1993), that relative clauses take up the Spec(DP) position in languages like Sinhala, Japanese, Korean and Basque, which do not show Complex island effects.