1. Code block is exited by double enter (so two new lines)
2. No shortcut for the checkbox unfortunately
3. Only Themes can change the fonts - see e.g. "Steel Blue" in the demo document to see how it's done. https://github.com/zadam/trilium/wiki/Themes
4. You could use custom CSS to change it however I can't recommend this: https://github.com/zadam/trilium/wiki/Themes#custom-css