MBA aluminum unibody prone to dents & dings

First of all, a ding that you can't even see would not make the machine fragile. Define small knock. What did you knock it against?

2nd, no that won't void the warranty unless you are way underselling your knock and you actually dropped it.

From my experience, no. I got my PBs and MBPs serviced with dings on them, it was only once I got the side grill on a PB pushed in quite a bit that they refused service.

That won't be a problem. Normal wear and tear won't cause internal parts to fail.

Small nicks should be fine. I've seen at Genius Bar people bringing BEATEN UP MBPs and most of times Apple is OK with it unless the machine shows signs of drop or crash. Aluminum is a quite soft material especially when it's thin, small impressions and nicks can happen quite easily.