[RPG] Using the Hitting Cover variant rule, is it impossible to hit the covering creature if its AC is 2 higher than the target’s

No. You get to wait to see if the attack would hit you.

It's specifically a reaction to when you were hit and it allows you to rewind and replay the attack as if you'd cast shield before it (using the original attack roll). See the text:

Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell

...

An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile. (Basic D&D p100)

So the clear intent here is the use it after the attack roll so you know whether or not you've been hit.

This is not meta gaming any more than 4e's immediate interrupts were meta gaming (if the action negated the trigger, then the action didn't happen but was expended). This is about modeling quick reaction in a way that's fair. It might be a bit meta, but it's far better than requiring a wizard to use a valuable spell (especially at early levels) for something that wouldn't have affected them anyways.

If you're looking for a simulationist model, this is the wizard waiting to see if the attack would otherwise hit them and only casting shield then.

This isn't nearly as powerful as you think. Let's assume that we have a squishy character like William the Wizard, his much tougher friend Bob the Barbarian, and they are fighting the evil goblin, Arthur the Archer.

Yes, Bob can move between William and Arthur. However, if Bob moves on his turn, Arthur can:

• Move on his turn before he shoots so that he has a clear path to William. This invalidates Bob's move at exactly the same price as Bob paid - one move. Obviously, this won't work if the only possible path is the one through Bob. (If Bob is standing in a doorway, for instance.)

• Arthur can just shoot Bob instead. This is probably what Bob wanted, because he'd rather take the damage than have William take it. However, the +2 to AC still isn't a consideration.

• Arthur can try to shoot William anyway. Here, he finally has to contend with the +2 to AC. Bob has made it 10% less likely that Arthur will hit Bob. It's hardly unbalanced, though; Arthur had other options that he ignored out of blind stubbornness, and 10% really isn't that much.

Now, if Bob really wants to give +2 AC to William rather than just taking the attack himself, he would probably Ready an action to jump in front of William just as Arthur begins to pull back his bow. (Or some similar trigger.)

• The first problem with this is that it's probably not possible. A generous DM might let you take a reaction in the middle of an attack, but most would probably say that Arthur still has time to change who he is targeting or even move before he takes the attack.

• The second problem is that assuming this works, Bob has used up both his action (to Ready) and his reaction for the round, and all he's gotten out of it was +2 AC against one attack for one ally. That's laughably inefficient, if not downright stupid. There might be an extremely specific situation where it was useful, but most of the time, it's just wasting Bob's action economy.

• Finally, it's worth considering that there is a much more powerful option for the character who wants to protect his allies in this way: the Protection Fighting Style. This allows you to impose disadvantage on an enemy attack (much stronger than +2 AC) using your reaction.