Separate environments for each project is the recommended way for most more-featured package managers.
They often don’t have quite the same they handle the specifics; however, they tend to be centered around that. For example, see the ’ Welcome Page’ and ’ Sessions and Projects’ sections of the JupyterLab Desktop 2023 update announcement. Or see ’ Working with Python Projects’ under the ’ Installing Python 3’ Guide in TalkPython’s ‘Training’ content. (While that last one doesn’t cover uv with Jupyter, you can see this thread or ’ One Line Command to Launch a Notebook with Pytorch’.)
However, one environment per notebook was suggested here.
If that means you have only the latest numpy available, that will be good for current development. However, it isn’t ideal as you may then cause problems for your old projects. (Hypothetically, a possibility. Hopefully, not the actual case. If you later find it is the actual case, then it is a further example of why I said earlier you outgrew just relying on a single installation on your machine. Any why environments are popular with Python coders.)
We may never know what triggered the issue. You talked about swapping back and forth between Jupyter 6 and 7 and so likely it was in there. Maybe ipympl would have worked earlier than that with a browser page refresh; however, you didn’t know to try that at the time and now have changed things around and dug yourself a deeper hole. Probably not worth exploring that aspect too much now. You just want to get back to something you can use your notebooks and code with again.
As to your issue with Jupyter:
Are you able to try a fresh install that would be separate from your own? For example, do you have access to a similar machine as yours? Or can you make a new user account and start fresh to see if ipympl will work in Jupyter there to give you interactive plots? (Maybe use JupyterLab Desktop there as test installation?) This would help you know if it is something about your system beyond what is going on in your own user account? I suspect it is just your user account and that you have old files around because Jupyter update can have issues on computers with older installations having been previously present and not cleaned out prior. (Those won’t necessarily show up in the listing of versions.)