I've got a Singleton class instantiated in Primary.swf situated at domainA. The file loads Game.swf from domainB via LoaderMax (http://www.greensock.com/). After I attempt to access the Singleton I suppose this really is different then the on instatiated in Primary.swf.
Can there be in whatever way of forcing this?
It ought to be exactly the same singleton in case your files are loaded in to the same application domain. You should use LoaderContext to achieve that.
Also, use a dependency injection framework, like SwiftSuspenders, to consider proper care of this more stylishly, consider many of these depend heavily on describeType, I'd advise to check for performance issues if you want to inject stuff into lots of game objects.
This is among the many issues with using Singleton (http://misko.hevery.com/2008/11/21/clean-code-talks-global-condition-and-singletons/). Might I would recommend that, rather, you just pass the instance into the overall game.swf, and then leave down to how that's instantiated to primary.swf?