Why are there varying vowels on proper nouns in Megillas Ester?

Question

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?

Answer 1

There are grammatical rules in general, but for your case:

1. It's always שושַן הבירה, otherwise שושָן.
2. It's always מרדכַי except on an esnachta, sof pasuk, or in exactly one case a zakeif katon ויגידו למרדכָי את דברי אסתר.
3. The general rule is any of the letters בגדכפת have a dageish (dot) at the beginning of a word, except when they come after a word that (a) ends in a vowel, like היא and (b) has a non-pausal note, like munach. (There are a few more exceptions to the exception.) In my examples, I'm thinking particularly of בערב היא באה.

In general, it's important to pay attention to the pronunciation, but of the three things you've asked the first one is the most important, because it actually changes the meaning of the word. שושַן הבירה means "the Shushan of the palace", and according to many commentaries refers to a different section of the city than just plain שושָן. (I still don't think it would invalidate the reading if you mess it up, but it has higher significance because of this difference.)

Answer 2

מרדכי takes a Kamatz when it is on a hard pause (same as most nouns that have a patach or segol on the primary stress).

שושן takes a Patach when it is attached to הבירה (same as any noun ending in a Kamatz in construct form).

באה loses it's Dagesh when the previous word ends in an open syllable and has a conjunctive Trop (same with any word opening with a ב).