LDAP should traverse the trust if the query is not domain specific. But
I think the query being used in the ADProvider is simply querying one
Domain, not the Forest (check the way your connection string is
established). It should be possible to do the whole forest, but I'm not
enough of an LDAP expert to tell you how it could be done.
ADAM will work. I've done LDAP authentication against an ADAM
directory. What I haven't tried is synchronizing ADAM against two
domains. That might work but I haven't tried it. I've normally used
ADAM for establishing Extranets where users will not be in the main AD.