Currently when you are setting up addressee and salutation you can select specific addressee/salutation or from configuration but both of them can only fall back to another addressee/salutation of the same type then to full name if they are blank. You should be able to select a specific addressee then if that one is blank have it next pick one from configuration.
If you choose "from individual" you may get your desired result if you have the formats on individual records. There's a second option there.
You may need to add some add/sal types to your records. Ex: Format for Joint Informal will pull Bill and Sue Smith. If formated correctly, it will also pull Joe Brown (no spouse on record) so I don't need a back-up add/sal type of Single Informal.
With all the variations, couples with different last names, etc. I can't see myself using the from configuration without having to go through the merged document very carefully.