Usually, dlcs are meant for existing customers.
What emergency. Did you even play all ME games without then with DLC?
Arrival was key setup of Shepard's indoctrination. You might still believe the kid through the next game was just his imagination, the point is, all but one mission DLC Bioware did in ME2 and ME3 held keys to understand the complete story.
The arrival ends on a ticking countdown.
The indoctrination is one interpretation. Mostly born from disatisfaction of the story.
ME3 starts with a classical narrative rope. Sheppard is set to be the galaxy saviour. On his path to his destiny, he meets with a vulnerable member of his species he fails to protect and save.
Plain character building. Sheppard is that guy who failed at saving a member of his own race (default Sheppard) yet will be given the opportunity to determine the fate of the galaxy.
The guy who can not wipe his own arse but will take care of everyone narrative trick.
It did not bite. Probably because writers spent three episodes building Sheppard as a character who achieved the impossible. Then the failure at saving a child of his own race did not fit well with people who associate closely with the character.
The boy was there at the start, the apparatus at the end appears with the same face to recall who Sheppard is: a guy who is going to determine the future for everyone while he was unable of saving a kid of his own race.
The arrival has nothing to do with that. Beside, it is the start that is said to be confusing. Even when going the indoctrination path, it does not click.