Hi ,
I took another look at this again today and noticed something that I had missed the last time. In the example project that you provided, you were using Interfaces as the parameters for your interface functions. This may have been contributing to some of the problems you were experiencing. I reconfigured the functionality that you had set up in the example project to use Actors as the parameters, and used message nodes to trigger the events.
Instead of using this:
I set it up like this:
Using this setup, I was able to play the game and fire projectiles without crashing, and I received a total of four messages each time I fired a projectile: one each from the Character and HUD events, and two from the Projectile event. Compiling either the MyCharacter or MyHUD Blueprints will cause the other to indicate that it is dirty and needs to be compiled, but they will both work fine.
I am not sure if we have done any testing with Blueprint Interface functions taking other Interfaces as parameters. I believe the original intent for Blueprint Interface functions was that they would take Actors as parameters. I have submitted a report to have this investigated further (UE-7920).