Can a single Shield of Missile Attraction curse an indefinite number of creatures

The Shield of Missile Attraction does not automatically attune to characters who use it. Attuning to it causes its "curse" to kick in, but it has no ability to automatically attune or force its wielder to attune. The point you might be missing is that unless you attune to it, you also don't get the resistance to ranged weapon attacks.

Certain magic items require a user to attune to them before their magical properties can be used.

The resistance to ranged weapon attacks is a magical property, and the Shield of Missile Attraction includes the "(requires attunement)" tag, so you don't get the "curse" or the benefit unless you attune.

You may have noticed that I put scare quotes around "curse". Yeah, that's an important point here. The shield grants you resistance to all ranged weapon attacks, so causing them to hit you instead of your allies isn't really a curse - it's the greatest benefit the shield has to offer.

RAW, B is targeted

One of the foundational rules for playing the game (found in the Introduction to the Basic Rules):

many racial traits, class features, spells, magic items, monster abilities, and other game elements break the general rules in some way, creating an exception to how the rest of the game works. Remember this: If a specific rule contradicts a general rule, the specific rule wins.

In this case we have the specific rule from the magic item shield of missle snaring contradicting the general cover rules:

A target with total cover can't be targeted directly by an attack or a spell

Regardless of the cover, the specific rule of the shield trumps any targeting limitations as the shield says (emphasis mine):

Whenever a ranged weapon attack is made against a target within 10 feet of you, the curse causes you to become the target instead.

Whether or not you are a valid target at the time of the attack is irrelevant as the specific magic item rule makes you into the target. No exception is provided by the specific rule (for what it's worth, it specifies "whenever").

How to narrate this

Total cover states that you can't be targeted directly by an attack. Perhaps the shield magically modifies the trajectory to take a path to you (similar to how fireball's effect can go around corners). If you are behind total cover, the arrow navigates around the cover.

A GM could then rule that your position relative to the new indirect trajectory has half cover, three quarters cover, or even impose disadvantage as he/she warrants. This is part of the procedure for Making an Attack:

Determine modifiers. The GM determines whether the target has cover and whether you have advantage or disadvantage against the target.

There is also an optional rule in the Dungeon Master's Guide called Hitting Cover that could aid in the narration:

When a ranged attack misses a target that has cover, you can use this optional rule to determine whether the cover was struck by the attack.

Essentially if the attack would have hit the target without the three-quarters cover (or whichever you impose) but the +5 AC makes it a miss, the cover blocks the arrow.

Rules as Common Sense

As a general disclaimer, a GM can rule that the cover provides substantial AC to block the arrow as a common sense ruling. Any GM can change the rules so the table can enjoy the game to their highest degree.