. are all three (four?) NAPs really being used
Yes. for some value of "used".
. Is there any evidence that the NAPs are really backing each other up?
Not sure this is possible. Perhaps the better question is, are providers using the NAPs to back each other up.
. do we have some regular examples from *any* site A initiating a connection from A to B, A to C, and A to D, where the three are verifiably (via traceroute, I guess) would traverse different NAPs (and hopefully only one each)?
Yes.
. Are there routing stability reports accessible online from the RA (or whoever else feels responsible for this) that graph fluctuations at the NAPs, including correlation among them? What are the quality metrics for routing stability?
Being defined.
. Do all the NAPs provide online statistics?
No.
. Are the NAP and RA regular reports to NSF publicly (hopefully via the Web) available?
http://info.ra.net/papers have the annual report/plan papers
. Is there any way NANOG can be used to exchange status information accessible via the network. Is someone already working on that? Would not NANOG be *the* forum to cooperate on that?
Sounds like a really good idea to me. --bill