IA's legality in general has apparently never been
tested in court,
A bit too generic a statement; I assume you're talking only of the
legality of giving public access to Wayback copyright-eligible all
rights reserved content.
IA follows a standard which is designed to avoid litigation:
http://www2.sims.berkeley.edu/research/conferences/aps/removal-policy.html
Until the Oakland Archive Policy is supersed, they're not going to
change their policies. Is there an alternative standard that one (e.g.
Wikimedia) could adopt? If not, who's going to make one? Probably
netpreserve.org and IFLA would need to be involved at least.
Nemo