Regarding 4a: if you lose your key (and thus you have lost the ability to disarm the alarm) the entire vehicle is bricked?
The assumption that this kind of thing needs to be done via the OBD-II port is wacky from the get-go. It may have been easier for some lazy system integrators at BMW to do it that way, but it certainly wasn't necessary, or apparently advisable.
The best practices in auto security are probably reflected by whatever the leading Japanese brands are doing these days. Traditionally Hondas have been the biggest theft targets, but a glance at the list of most-stolen cars in America ( http://editorial.autos.msn.com/article.aspx?cp-documentid=43... ) suggests that they've more or less solved the problem, as of the mid-1990s. I seriously doubt there's that much need for further innovation.
This means that a manufacturer can't be the exclusive source of resetting a key much in the same way that Verisign isn't the exclusive source of SSL certificates. Due to the anti-competition legislation, you should be able to take your vehicle to any local garage and have it fully serviced whether for a tune-up or to get a key reset. And if a local garage can be employed to reset your key because they have access to the private key required to sign the key request, thieves can do it just as easily.