Apple Employee damaged my MacBook - What should I do?

Nothing is worse than bringing something to someone and having it returned in worse condition than before. The nincompoops who assisted you are obviously need of customer service training not to mention an infusion of some common sense. The carelessness you describe should not be tolerated.

There is a very specific test to perform to verify the display hinge clutch operates within tolerances. It is performed by releasing the lid so that it closes from a specified position, not opening it so quickly that it slams against something else. The Geniuses ought to be familiar with it. Did they do that, or not?

Edit to add: the black plastic hinge cover is just a cover. It's OK for it to be somewhat loose and for there to be variations among them. Separate components comprise the actual hinge mechanism. They cannot be inspected without disassembling the MBA.

So what should you do in this circumstance? The same as you would do in any other. Write to the store manager describing what happened and ask what Apple intends to do about it. You probably won't get very far at first. Be persistent. Short of external pressures that only you can bring about, that's all you can do.

P.S. can I ask Apple to check the cameras in the store to back my evidence? I have a recording of the whole experience, and you can hear the macbooks hitting each other when the manager opens the lid.

You can ask, but those cameras are Apple's property to do with as Apple wishes and to protect Apple's interests. They don't install them for you to record evidence that might be used against them.

So I said okay, and asked for them to run a virus check because of a pop-up that appeared on my screen stating I had a virus. Now my original Genius gave me an attitude trying to explain the difference between adware, pop-ups, and basically teaching me the differences.

Those scams are very popular and should be summarily disregarded. A "virus check" is unjustified and never appropriate anyway. To learn more about the popup that appeared and what to do about them, read Phony "tech support" / "ransomware" popups and web pages.

Adware is a very different threat, and requires your active intervention to install. To learn how to recognize adware so that you can avoid it, read How to install adware. Should you ever find yourself affected by it, Apple's instructions are linked in the Recovery Procedure near the end of that document. You don't need a Genius, certainly not the kind you encountered.

That's awful. Are you sure you didn't confuse a muffler repair shop with the Apple Store?

You expressed your concerns patiently and in specific detail, but they ought to be sent in writing, in the same manner, to someone with the authority to address them. All the rest of us can do is lend a sympathetic ear.

Write to the store manager by finding its address here: http://www.apple.com/retail/storelist/. Send a copy to their corporate address.

The manager opened my macbook's lid too quickly and too far, that's the reason it hit the macbook behind it, not because of the clutch.

I am envisioning a situation in which the technician opened the lid using a force she would consider normal, and was surprised when that force caused it to fly open unexpectedly quickly, resulting in the MacBook behind it being struck by its lid. That's my interpretation of your report. Having seen what occurred yourself, you are in a better position to understand exactly what happened.

These people open and close MacBook lids all day long, so I don't know how else it would have occurred other than yours being excessively loose.

I'm not a very confronting person.

It may be unpleasant but sometimes that's the only way to get results. Your MBA may have suffered only cosmetic damage, but it was the result of negligence inconsistent with any reasonable standard of care.