I don’t know what that means. You still did not answer the questions.
• the player creates the UI, great
• how does the player get into the game (placed in the scene, spawned with a node, spawned by the game mode?)
• how does the BP Spawner actor instance get into the game?
This is not an issue with an Interface. This is never an issue, neither is casting. This is an issue with referencing. You say you know how to pass variables between BPs, but the widget is not even referenced, not can we see a dispatcher.
Also, if you feel you must use Get(All)ActorOfClass, things are already not great communication wise. At least not for something like this since the nodes exist for a reason and can be handy. They are just misused or used instead of communication. It’s a very short lived “solution” which creates a lot of confusion and stops working rather quickly.
This is an even bigger issue. Widgets are volatile, their role is to show stuff, handle UI and player input. The very last thing you want user interface to do, is to run logic. Even if the game is just an interface (like a puzzle game), do consider avoiding it.