Summary:
We know that רשב"ג and ר"א are not discussing an ערער חד, since ר"י stated concerning their מחלוקת that אין ערער פחות משנים.
This ruling of אעפ"מ applies (only) if there is a חזקת כשרות, even if there is no עד המכשיר; similar to the case of מעלין לכהונה where the עד המכשיר and the קול cancel out each other.
And a rumor was spread about him; and we demoted him - ונפק עליה קלא ואחתיניה
Summary:
There is a זילותא דב"ד even by a single reversal. 'אחתיניה' however, means that the individual stepped down from the כהונה on his own. Alternately there is זילותא only when a פסק based on עדות is reversed; but not a פסק based on a קול.
Summary:
In a case of תו"ת, the ruling מן התורה is, that we follow the חזקה, whether there is a חזקת היתר or a חזקת איסור. The חכמים however instituted that even if there is a חזקת היתר, it should still be treated as a ספק. If the issue is a דאורייתא, we go לחומרא if it is a רבנן, we go לקולא.