Why was Esther permitted to Mordechai up untill the Feasts with Achashveirosh?

Question

The Medrash tells us that Esther was the wife of Mordechai, and once she was forceably taken to be the queen and Achashveirosh's wife, she was still permitted to Mordechai because she was taken Beones. Once she went to Achashveirosh to invite him to her first meal, she was no longer permitted to Mordechai because she went willingly. (this being the Midrash's explanation of "Ka'asher Avaditi, Avaditi"⁽¹⁾)

It is perplexing that she was still permitted to Mordechai at that point, because earlier she willingly saved the life of the man who was raping her, by telling Achashveirosh about Bigson and Seresh. At that point she should be considered "willing" and prohibited to Mordechai?

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⁽¹⁾ _{Gemara Megila 15a: R`Abba said: It will not be [she said] according to the custom of every other day. Till now [I have associated with Ahasuerus] under compulsion, but now I will do so of my own will. And if I perish, I perish. As I am lost to my father's house so I shall be lost to thee.}

Answer 1

I found in Ramban, Milchamot Hashem Sanhedrin that this Midrash follows the opinion of Abaye that the single permission for Esther was Karka Olam, not Hanaat Atsman:

דמאן דגריס באגדה דאסתר אשת איש הואי כדאמרי' שהיתה עומדת מחיקו של אתשורוש וטובלת ושוכבת בחיקו של מרדכי ודריש כאשר אבדתי מבית אבא אבדתי ממך סבר (עיין מסכת מגילה דף ט"ו עמוד א') לה כאביי וטעמא דקרקע עולם דההוא טעמא שייך אפי' בא״א אבל רבא דפריש משום הנאתן ליתיה לטעמא אלא בשאר עבירות דפרהסיא ובגמ' לא דייקינן בה באישות כלל דהא מסיימא לן ולא סבירא לן אגדתא דרב ירמיה בר אבא דהתם

So when she was forced it was "din Ones", but when she get alone to Achashverosh it was not "Ones". A married woman, when her husband is not Kohen isn't prohibited to him if rapped, but if the sexual intercourse was not a rape, she becomes prohibited to the husband. Ramban continues and say that from an Halachic point of view, we don't follow the opinion of this Agada and if the lover is a non-Jew the wife remains allowed to her husband despite that the sexual intercourse was not a rape. Tosfot Sanhedrin 74b discussed also this Aggada. It seems that Rabenu Tam will agree with the opinion of Ramban that this Aggada is not relevant from an Halachic point of view.

But the main question was about the fact that she was permitted before this event. The answer is that she was beOness following the opinion of the Aggada. This is true from an Halachic point of view, according to all Rishonim (not Ramban and RT only, but also Baal Hamaor and Rabbi yitschak Bar Mordechai in tosfot Sanhedrin 74b)