Regarding Mahalya Paksham: Must we prepare the mahalaya paksham shraddha diet during the entire period? Or can we follow the usual (non-Mahalya Paksha) preparations after Mahalya tharppanam (post Madhyashtami)? Should the married couple take head bath on all the 15 days?

No need to prepare shraddha diet for the entire duration of Mahalaya Paksham. Only the day when Mahalaya Paksha tharppanam is performed, and on the day of Mahalaya amavasyai, shraddha diet is prepared.
If Mahalaya Paksham tharppanam is done on all the 15 days, both the couple must take head bath on all 15 days. When tharppanam is done only once during the 15 days, it is enough to take head bath on that one day of tharppanam.