First, you can simply add the address to your recipient policy, and RUS will add it to all the mail accounts on the server. You can also set it as the primary address. Then, when youa re ready to make the switch, have your primary MX record point the server for mail, and give your current primary a lower preference, so mail can still be delivered under the old domain name (believe me that you will be recieving mail at the old domain for quite some time).
To you, you have a BDC. To Windows, it is simply another DC. It is not recommended that Exchange be loaded on a machine that has active directory installed, but it can be done, though you may have some problems with it, mostly little things that are of no real consequence. If youare indtending to use it as a backup server only, get a copy of DoubleTake or anothe replication software package that can replicate Exchange and install that on both servers and start the replication. However, this will not resolve your issue of creating amailbox when adding a user. What you probably need to do is to make the AD on the machine a Global Catalog. If you do go ahead and install Exchange, you will need to give the instal a different name, which ca be changed if you need to bring up this server as a replacement for your current Exchange server.
I do not know of anyone imaging a DC or Exchange server. A failure on either is usually handled with a rebuild and a restore from backup.
As for the lack of an NDR to a user, check that there are no rules enabled that may be acting upon recieved NDRs. I don’t know if you can use message tracking or not on NDRs, I’ve never tried it, but it may be worth a shot to see where the NDR is going.