It is a known Gemara that the Jewish people "re-accepted" the Torah during the times of Esther.
Question 1. Why did the Jews of that time specifically merit to be the ones to re-accept the Torah? Had there been no more deserving period of Jewish people prior to the story of Purim?
Question 2. In what way did they re-accept, did their practical observances change in any way?
Question 1. Why did the Jews of that time specifically merit to be the ones to re-accept the Torah? had there been no more deserving period of Jewish people prior to the story of Purim?
In the Maamar Ve'atah Tetzaveh, the Rebbe explains that every exile present challenges with regard to observance of the Torah and mitzvos. In the time of Haman, those challenges were great:
In the times of Achashverosh, by contrast, the Jews experienced the ultimate of descents. Every exile is associated with a veiling and concealment of G‑dliness. {For all exiles follow the paradigm of the Egyptian exile,28 of which it is written,29 “They did not listen to Moshe because of their dwindled spirits and hard toil.” Similarly, [all subsequent] exiles present several challenges with regard to [the observance of] the Torah and its mitzvos.} In particular, then (in the time of Haman), there was an even greater veiling and concealment [and the very lives of the Jewish people were endangered].
Nevertheless, the thought of anything outside [the context of our faith], heaven forbid, did not occur to them.)
Moreover, they exhibited self-sacrifice in the observance of the Torah and its mitzvos to the extent that they congregated to study Torah communally with self-sacrifice.}
It was Mordechai, the Moshe of the generation, who inspired this self-sacrifice. [On this basis, we can understand the verse] “The Jews accepted what they had already begun,” that the giving of the Torah was merely a beginning and their acceptance came at the time of Haman’s decree. For their actual expression of self-sacrifice in the observance of the Torah and its mitzvos elevated them (in this regard) to a level above that experienced at the giving of the Torah. Therefore, this was when the acceptance of the Torah took place, “the Jews accepted.”
Thus, the Jews were willing to sacrifice their life for their faith. As the Rebbe explains further in the maamar.
The Rebbe explains that for example, during the Chanukah-story, only a small part showed mesirus nefesh. However, during the Purim-story, the whole of the Jewish people showed mesiras nefesh and took upon theirselves to accept the Torah, even though it was very dangerous to do so at that time.
at the time of Haman’s decree, the entire Jewish people displayed mesirus nefesh
The Frierdiker Rebbe, the previous Rebbe, explains in the Maamar VeKibeil HaYehudim 5687 (1927) that:
In the time of exile, everyone is broken and crushed - but it is precisely at such a time that one reaches the [essence of the] luminary itself. This in turn allows us "to keep a constant light burning."
In the time of Purim, there was a huge ammount of mesirus nefesh, as the Midrash Rabbah in Esther Rabbah 9:4 says:
After he erected the gibbet, he went to Mordekhai and found that he was sitting in the study hall with the children sitting before him, with sackcloth on their waists, engaging in the study of Torah, and they were screaming and weeping. He counted them and found there twenty-two thousand children. He cast iron chains on them and deployed guards over them. He said: ‘Tomorrow, I will kill these children first and then I will hang Mordekhai.’ Their mothers were bringing them food and water and saying to them: ‘Our children, eat and drink before you die and don’t die hungry.’ Immediately, they placed their hands on their books and took an oath: ‘By the life of Mordekhai our master, we will not eat and drink, but will die while fasting.’ All of them began weeping loudly until their cries reached the heavens. The Holy One blessed be He heard the sound of their weeping approximately two hours into the night.
How would a person respond when someone treatens him with death if he studies further? The people did not stop studying Torah. No, they continued, accepting the risk. Why? Because the Torah was way more important than their lives. They wanted to live with G-d, no matter what!
The Etz Yosef in his commentary to this Midrash actually links the number of students to Matan Torah.
ושנים עשרים אלף תינוקות. [יש מדרשים אומרים שהכונה בשנים ועשרים אלף תינוקות לשנים ועשרים אותיות באל"ף בי"ת ורצה לבטל את מרדכי ותלמידיו מלעסוק בתורה (יפה ענף ועץ יוסף מהדורא קמא)]. ולי נראה שקבץ כמספר הזה שיעסקו בתורה כנגד המלאכים שבאו עם הקב"ה להר סיני בנתינת התורה לישראל כדכתיב רכב אלהים רבותים אלפי שנאן אדני בם סיני בקדש. וקיבץ תינוקות סך הנ"ל משום שיהיה הבל פה שאין בו חטא כלל:
This whole question could also be linked to the question whether Matan Torah was out of coercion or out of love. See my question here, with fascinating answers. The Tosfos on that famous Gemara says:
But at the time of Achashverosh, they accepted [it] with their consent from love of the miracle.
See similary, the commentary of the Ohev Yisrael (Parashas Zachor and Purim), on the significance of "accepting out of love":
But even so, they accepted it in the days of Achashveirosh, which was a time of great pain and distance from G-d, and even so, they accepted it upon their souls, even though they were in a very lowly state, and committed to do all that was written in the holy Torah. And so we find that there was no longer any mitigating claim about the acceptance of the Torah after they accepted it out of the desire of their hearts.
And, the Emes LiYaakov writes (אמת ליעקב - ברכות, זרעים, מועד):
שבאמת צריכים לשני הענינים, היינו העבודה מאהבה והעבודה מיראה - Sometimes we need worship out of love and also out of fear
B"H when studying the maamar, I've stumbled upon another explanation on why the generation of Mordechai was special (enough) to say that the re-accepted the Torah. In describing Mordechai, the Torah (Esther 2:5) uses the words "Ish Yehudi". The Midrash (Esther Rabbah 6:2) says that Moshe Rabbeinu was also called "ish" and thus, the Midrash says:
אִישׁ מְלַמֵּד שֶׁהָיָה מָרְדֳּכַי שָׁקוּל בְּדוֹרוֹ כְּמשֶׁה בְּדוֹרוֹ - ish teaches that Mordekhai, in his generation, was the equivalent of Moses, in his generation
The maamar then goes on to explain that:
the unique quality of Mordechai was that he served as a shepherd of faith for all the Jews of his generation [...] drawing down knowledge to all the Jews in his generation.
The Midrash goes further:
Just as Moses taught Torah to Israel, as it is written: “See, I have taught you statutes and ordinances” (Deuteronomy 4:5), also Mordekhai did so, as it is written: “Matters of peace and truth” (Esther 9:30), and as it is written: “Acquire truth and do not sell” (Proverbs 23:23).
See the commentary of the Yefeh Anaf (ad loc.).
The Rebbe connects the greatness of the Purim-generation to the revelation on Sinai (letter from the Rebbe, dated from 11 Teves 5718):
Only Jews who have faithfully adhered to the Torah and Mitzvoth, as they were revealed on Mount Sinai, have survived all their persecutors, for only through the Torah and Mitzvoth can the Jewish people attach themselves to the Superior and Supreme Power, G‑d, who has given us the Torah and our way of life.
I hope this answers your first question.
The Ramban on that Gemara explains that as long as they were in Eretz Yisroel they had to keep the Torah because Hashem gave them the land, even if the acceptance was coerced. However, once they were in galus they were able to claim they were free from keeping the Torah since they no longer have the land.
ונ"ל לומר, דמתחלה אף על פי שהיה להם מודעא מ"מ לא נתן להם הארץ אלא כדי שיקיימו התורה כמו שמפורש בתורה בכמה פרשיות, וכתיב ויתן להם ארצות גוים ועמל לאומים ירשו בעבור ישמרו חוקיו ותורותיו ינצורו, והם עצמן מתחלה לא עכבו בדבר כלל ולא אמרו במודעא כלום, אלא ברצון נפשם מעצמם אמרו כל אשר דבר ה' נעשה ונשמע, (לפי) [לפיכך] כשעברו על התורה עמד והגלם מן הארץ משגלו מסרו מודעא על הדבר מדכתיב והעולה על רוחכם הי' לא תהי' אשר אתם אומרים נהיה כגוים וכמשפחות האדמה לשרת עץ ואבן, וכדאמרי' באגדה (סנהדרין קה.) רבי' יחזקאל עבד שמכרו רבו כלום יש לו עליו וכו' לפיכך כשבאו לארץ בביא' שניה בימי עזרא עמדו מעצמם וקבלוה ברצון שלא יטענו עוד שום תרעומות, והיינו בימי אחשורוש שהוציאם ממות לחיים, והי' זה חביב עליהם מגאולה של מצרים:
Based on this we can understand why they were the first ones who accepted it willfully.
The Pnei Yehoshua explains further, that by Har Sinai they said naaseh v’nishma on Torah shebiksav, but not Torah Shebal pe. It was on this that Hashem forced them to accept with the mountain. (I believe there might be a Tosfos somewhere that says this as well, certainly there are others who mention it as well). After the Purim story, they had the first opportunity for a new mitzvah drabbanan, and when they realized that it was hinted at already in the Torah they accepted Torah shebal pe willingly.
ועוד היה נ"ל דעיקר התנאי במעשה בראשית לא היה אלא על תורה שבכתב אבל כפיית הר כגיגית לא היה אלא בשביל תורה שבעל פה שבשבילה כרת הקב"ה ברית עם ישראל כדאיתא בפרק הניזקין והיינו מה שאמר הקב"ה למשה וגם בך יאמינו לעולם ודרשו חז"ל שבא לרבות הנביאים והזקנים שקיבלו ממשה התורה שבע"פ לזה הוצרך כפיית הר כגיגית אבל בימי אחשורוש שהאיר הקב"ה עיניהם שכיוונו ברוח הקודש קריאת המגילה שהיה נרמז בכתב הלוחות כדאיתא בפ"ב דמגילה דף י"ט ע"ב דהא דכתיב ועליהם ככל הדברים וכו' מלמד שכל דקדוקי סופרים אפי' מקרא מגילה הכל מפורש בשום שכל בהלוחות ומש"ה מסתמא כמו כן קיבלו עליהם כל התורה שבע"פ כמו תורה שבכתב.
Based on this, we can understand that the practical way they accepted it willingly was by undertaking the mitzvah drabbanan of Purim.
I once heard a shiur from R’ Mendel Kessin on this topic, and he explained this idea in detail. He added (if I recall correctly, based on a Ramchal) another thought, that originally they thought that Torah shebal pe was just a way of running a nation in the practical sense. Just like other nations have laws, they thought Hashem wanted them to have these specific laws. That it was basically a set of laws to run society based on the Torah, but not really a part of the Torah itself, and this is what they didn’t accept willingly. However once they went to galus and saw what the nations of the world are capable of, killing out an entire nation in one day, they realized that Torah shebal pe wasn’t merely just a different set of laws, but rather it was a unique part of the Torah itself. Therefore they accepted it willingly.
