Hmm, the setting would provide a lot of good logic supporting different nations (from different Shadows, in this case) coming together. The map itself would probably work best if it represented a single Shadow instead of trying to replicate some kind of multi-plane effect.
Make each nation a Prince of Amber to be their Pretender and it should work out fairly well, I'd think.
Oh, and I call Benedict if it gets tested in MP. Heh!
Another option might be to make the Princes and Princesses the various Pretender options for one nation and set up the scenario to be a conquest of a Shadow realm, as apparently often happened in the days before the Patternfall War.