"Have you tried the spotlight test with keyboard clicks ON?" Yes, many times. Additionally, I did this test in an Apple Store on 5 Air 2s. One clearly had the popping problem and popped frequently when you did this, the other 4 I tested did not.
"it's hard for me to believe you can replicate a hardware issue by changing options in iOS." You are turning off a specific sound when you do this. Turning on the keyboard has been shown to cause the pop. Other people have reported that their units popped at other times though. It seems reasonable to me that if there is a problem with some of the sound related hardware that turning off a sound would reduce its occurance. If it was software, I would expect my new unit, that had all of the orginal software restored from backup, to have the same problem. It does not.
"there are a few who've swapped devices and no longer had the issue. But, there are many more who have replaced their devices multiple time and still have the pop." People who swapped their unit and received another popping unit are much more likely to post again (repeatedly) than someone whose problem was resolved. There are many people who posted their problem and never posted again; I assume they either got a unit that doesn't pop or received a refund and walked away. It's very difficult to speculate on how many units have the problem based on this thread alone. I do think there would be a lot more people posting and a lot more new posters if the majority of units sold popped.