In those days, while Mordecai sat in the king's gate... (2:21)
In ancient times, it was customary for judges to sit in the gate and conduct their duties there (see i.e. Devarim 22:15 and Shmuel Alef 4:15-18). What was Mordechai doing sitting there? Perhaps this verse tells us that from the beginning of the story Mordechai was already a powerful member of the king's court, with the power to judge other members.
If so, then it is easier to understand why Mordechai thinks he can get away with not bowing to Haman, and easier to understand why Mordechai in particular is appointed second to the king at the end of the megillah.
This approach is not contradicted by the episode in which Mordechai reports the plot against Ahashverosh. Contrary to popular opinion, Mordechai probably did not overhear the plotters himself. Rather, "became known to Mordechai" presumably means that the matter was told to Mordechai by someone lower than himself, and of course Mordechai had the connections needed in order to inform the king.
The king loved Esther more than all the women ... so that he set the royal crown upon her head, and made her queen instead of Vashti. The king made a great feast unto all his princes and his servants - Esther's feast - and he made a release ['hanacha'] to the provinces, and gave gifts, according to the bounty of the king. (2:17-18)
"There is a certain people scattered abroad and dispersed among the peoples in all the provinces of thy kingdom; ... and it is not worthwile for the king to suffer them ['lehanicham']." (3:8)
In the megillah, the verb "lehaniach" here is used only in these two verses. The meaning not the same: Haman's "hanacha" means passive acceptance of the Jews' existence, while Ahashverosh's means some extra gift which was granted to the empire. The use of the same verb, with different meanings, within a space of 12 verses draws attention to the contrast between the uses.
It appears that the when all the empire's peoples received Ahashverosh's "hanacha", the Jews were among them. No doubt they were happy at receiving gifts, at the opportunity to party in Shushan, and for those who knew, at the knowledge that one of their own had "made it" by marrying into the royal family. Mordechai's loyalty to Ahashverosh when he learns of the plot (2:21-23) is an indicator of this identification with, even assimilation into, the ruling establishment.
At this exact moment, the megillah uses the verb "lehaniach" again in quite the opposite context. Just when the Jews became most successful and integrated into Persian society, the question arose whether they even possessed the right to live. By connecting these two events through their juxtaposition and the common use of "lehaniach", the megillah suggests the transience and insignificance of their accomplishments in Shushan. If the accomplishments could be uprooted so quickly by the very people who had granted them, then perhaps they were not worth putting so much effort into in the first place.
Esther spoke again before the king, and fell down at his feet, and cried and begged him to eliminate the evil of Haman the Agagite... (8:3)
According to the gemara (Megillah 14a) we do not say Hallel on Purim because despite the miracles at the time we remained slaves to Ahashverosh. We still must commemorate being saved, but this cannot be an opportunity for complete rejoicing.
The crisis in the Megillah began when Mordechai refused to bow to Haman. In verse 8:3, we see that the crisis is ended by Esther bowing to Ahashverosh. Perhaps it is somewhat better to bow to Ahashverosh than to Haman. But whatever principle was gained by Mordechai's initial act is likely lost here. This is a vivid illustration of the gemara's point that it was not yet the right opportunity to say Hallel.
For writing which is written in the king's name and sealed with the king's ring cannot be reversed. (8:8)
One interesting feature of the megillah is how Ahashverosh cannot simply cancel his decree to exterminate the Jews. Instead, he can only issue a complementary decree allowing the Jews to fight back. Perhaps another example of inability to cancel a decree occurs after Ahashverosh divorces Vashti in a fit of drunken rage. Once he sobers up he cannot reverse this decision, but must search for a different queen.
We see the same phenomenon regarding a different Persian king in the book of Daniel (6:7-17). King Darius decrees that anyone who prays to God or a man other than Darius must be killed. Then Daniel is caught praying, much to Darius' distress since he likes Daniel. But his advisors tell him, "Know, O king, that it is a law of the Medes and Persians, that any prohibition or law which the king establishes may not be changed." Thus Daniel cannot escape being thrown into the lion pit, though in the pit he is miraculously saved.
Thus it seems that that in the Persian empire, a formal decree once made by the king could never be reversed. There is no evidence of a similar rule in other ancient kingdoms, and the logic behind the rule is not obvious.
R' Yaakov Medan ("Daniel: Galut vehitgalut", p. 152) suggests a possible rational for this practice. The 127-province Persian empire was so large and diverse that unless the law was held in the highest possible esteem, the empire could descend into anarchy. Making each royal decree permanent was a way of increasing its standing and the respect all the diverse peoples would have for it, thus decreasing revolts and making the empire more stable.
I hesitate to advance ideas that people much more informed than myself may have considered and rejected, but the following justification seems to me more compelling than R' Medan's. Presumably the unique legal system of the Persian empire reflects some unique quality of the empire itself. The Persian empire was not unique among ancient kingdoms not in its size, but in that it was ruled jointly by two distinct nations. The Assyrian empire was ruled by Assyria, the Babylonian empire by Babylon, and so on. But the Persian empire was ruled by both Persia and Media. Thus Ahashverosh's banquet is for "the army of Persia and Media" (1:3), and the above quote from Daniel referes to the "law of the Medes and Persians", since both nations are part of the government hierarchy.
The presence of two separate nations in power probably led to tensions between the two. In order to prevent power struggles between them from leading to civil war, some kind of restrictions on the government were necessary. There was likely a legacy of past royal decrees establishing the division of power between Persia and Media. By making these and all other formal decrees irrevocable, the balance of power between Persians and Medes was ensured, and the empire's overall stability was enhanced.