Okay, as an update: I received the new MacBook Air this morning, which is quite remarkable. Apple promised to expedite the replacement when they processed my call on Saturday, and here it is Tuesday afternoon and I've already had a chance to check it out. Absolutely amazing.
That said, the keyboard on the new unit is much improved. It's more solid, and now I realize that the previous one was simply defective. Yes, there's a tiny bit of flex on the new one if I look for it (and press pretty hard), but the keys are MUCH quieter, there's no stuck keys, the feel is smoother and again, more solid, and in general it's what I liked about the keyboard when I checked them out in the stores.
Also, the trackpad on the new one does not have the rattling/broken spring sound when I press on it, so that's a plus. This one also booted without any problems, and I didn't have the App Store issues I had with the previous unit. Heck, this one even has the Samsung LCD to go with the Samsung SSD, compared to the LG LCD of the previous unit (which I had found to be just fine).
So, yes, that first MBA was a bit of a lemon, but this one seems perfect so far. Suffice it to say, I'm now a very happy Apple customer.