"The second creature takes fire damage equal to your spellcasting ability modifier."
Spirit Shroud only takes effect on an attack, and as written the second creature "takes damage"; you do not explicitly make an attack against the second creature for the Spirit Shroud to affect.
The DM is obligated to describe what the character perceives. A level 1 adventurer is by no means "seasoned". Tagram the Great, also known as Tagram the Just, the Kraken Slayer, is not actually experienced in how to slay a kraken when he begins his adventures. You darned well cannot tell the difference between your arrow bouncing off of strong ...
The spell ends on each stone after the first attack.
Magic stone says:
Hit or miss, the spell then ends on the stone.
So the second (ricochet) attack will be a ranged weapon attack using the usual to hit (+prof+dex+1) and damage (1d4+dex+1) of the sling, not a ranged spell attack and damage of magic stone.
I'd argue it depends on the player's intent with the trigger.
If the wizard player asks to specifically blast the first enemy that comes through the door, they can't change that. The moment that door opens and reveals an enemy, they have to blast it or lose their reaction.
If they want to blast any enemy that comes through the door, they can choose to hold ...
Rules as Written
Consider the case where the caster readies, as Marq says, to "blast the first thing that comes through the door". With that as their trigger, the caster gets only one chance. When the first thing comes through the door, they can react to the trigger or ignore it. If they ignore it, however, they have now lost their readied ...
It seems intended to work
As Apocolisp indicated in their post it is indeed up to the DM on how to interpret that rule. I can see it both ways but leaned to Elemental Adept working in your example.
That said, although JC's Tweets are not considered gospel truth anymore (and for somewhat good reason), it seems intended that should and would work.
As shown in ...
Knowing somebody's location does not mean you can see them
The rules on Unseen Attackers and Targets state (emphasis mine):
Combatants often try to escape their foes' notice by hiding, casting the invisibility spell, or lurking in darkness.
When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're ...
They can get the location right, but attacks will still be made at disadvantage, and attacks against will still have advantage.
The rules for unseen attackers and targets are concerned with whether or not a creature is seen, not if its location is known:
When you attack a target that you can't see, you have disadvantage on the attack roll. This is true ...