Shehecheyanu on Purim
What is the shecheyanu for on Purim night?
The one before the morning megillah reading is (according to the announcement in shul) for the mitzvos done in the daytime. But in the preceding night, what does it refer to?
If it is in commemorating the mitzvah of megillah, then we are not fulfilling that mitzvah until the second reading as (I thought) the obligation is to hear 2 readings.
If, for some unfortunate reason, I am unable to hear a second reading wouldn't a bracha on the performance of the mitzvah have been in vain? And even if it is completed wouldn't there have been a sizable hefsek?
In terms of the daytime blessing, how can it cover multiple distinct mitzvot and what if one ends up not performing them?