Summary:
There is זילותא דב"ד (only) when the original ruling was based on עדות (but not if it is based on a קול). We are מבטל a קול if there is no remedy or if it is only a דרבנן.
We lowered him, we will elevate him - אנן אחתינן ליה אנן מסקינן ליה
Summary:
רש"י maintains that by תו"ת we follow the חזקה, but only regarding the person in question; the חזקה of the mother does not apply to the child.
ר"ת agrees that we follow the חזקה, however if it is בחזקת היתר and it is a דאורייתא issue the חכמים ruled that it is אסור מספק ; however if it is a דרבנן issue then it is מותר. The חזקה of אשה דייקא ומינסבא negates the חזקת איסור .
However the ריב"ב maintains that the effective חזקה is the one established by the first עדים.