I found another reason for why we went Single Threaded rather than muti-threaded. We have been working to integrate the Terrain 3D addon into our web projects. It now supports the web but currently only supports the single threaded version of Godot in the web. Now hilariously we are not using any Terrain 3D in this project. So I may simple remove and disable that to have a multi-threaded version.
I do not yet have the new build ready yet, but I have been working on my framework for the last week to re-write it to better handle loading and scene swapping. Your discussions with me really helped guide this re-design. I now have it smoothly supporting loading and swapping of scenes both single and multi-threaded. I also implemented your concept of Hot Scenes which I have named Stored Scenes, and now the framework can load multiple scenes into memory, while only assigning 1 as the current scene. It can also swap scenes in a single frame if the scene is a Stored Scene. This should make loading much faster even in a single threaded environment. Also finally I made sure that if an error in scene loading does occur, we now catch the error as best I can to let the user know and take them to an exit screen.
Again I just wanted to say thank you for helping me think through a better way to design our game framework and I hope it will result in better games from us in the future.