Your "whitelist" idea for the one or two allowed email addresses will work just fine.  SpamFilter checks all whitelists first.  A message that matches on ANY whitelist condition will be immediately accepted without checking any further conditions.

As for blocking all other messages addressed to that domain, there are at least two ways of accomplishing this.  The simplest (and fastest, since this is the first "blacklist" that SpamFilter checks) would be to simply remove that domain from the list of Allowed Domains.  The method you mentioned in your post would also work, but it does require slightly more processing on the part of SpamFilter, since the "local emails TO blacklist" is the fourth blacklist filter that is checked.  (See http://www.logsat.com/spamfilter/forums/showmessage.asp?messageID=2720 for an outline on how SpamFilter checks the blacklists.) 

If you run a fairly low volume mail system, it really doesn't matter, either method should work.  If you're processing 10,000+ messages per hour, then using the Allowed Domains for this filtering instead of the Local Email TO blacklist could make a significant difference in processing requirements.