In Megillas Ester, several proper nouns (such as Mordechai, and Shushan) use komatz and patach interchangeably. In sefardi pronunciation this does not make much of a difference, but in ashkenazi pronunciation, this makes names sound noticeably different: MordechAi vs. MordechOy; ShushAn vs. ShushOn. Is there a hard rule that rules the correct nikkud for these?
In addition, there are instances where the word "she came" has varying vowels with letter Beys and Veys, such as to make these words sound "B-a-a" "V-a-a." What is the rule there?