[jasongrout]

> How do I start a terminal-based REPL/shell that is sharing the same kernel as the notebook? (Relevant to text editors, because this might be for example an ipython shell running inside emacs, allowing me to easily evaluate fragments of code in the text editor.)

FYI, if you want to do both things inside of JupyterLab, you can easily start a console in JupyterLab connected to the same kernel as the notebook in three different ways: right-click in the notebook and select "New console for notebook". Or from the notebook, select the main menu File>"New Console For Notebook". Or simply start a console from the File>New menu and choose the notebook's kernel from the dropdown.