I'm not sure that I completely follow your configuration steps above, but I believe that I understand what you need to do.
If you would like:
x.x.104.112:443 ->192.168.0.12 (web server) and
x.x.30.26:443 -> 192.168.0.2 (SBS)
a) NAT > Port forwarding directed to and enabled for port 443 to 192.168.0.12
b) DMZ > Software DMZ enabled for directing traffic from x.x.30.26 to 192.168.0.2
I've just tried the above and it appears to be working correctly for me (I tested using port 21 rather than 443, although that shouldn't make any difference)
While the config above should work correctly, it will expose the SBS server to communication on all ports. Assuming that this is not what you want, you should also create some advanced firewall rules to restrict traffic to only permit traffic on port 443 through to the SBS. See this thread for more details, but in summary you just need two rules as follows:
1. From WAN1 to LAN1, source 0.0.0.0/0.0.0.0 dest 192.168.0.2/255.255.255.255 proto TCP sport any dport 443 permit
2. From WAN1 to any, source 0.0.0.0/0.0.0.0 dest 192.168.0.2/255.255.255.255 proto any sport any dport any deny
Note that you will be filtering traffic on the local (private) address, not the external address.
Hope that helps,