1) The plugins can be anywhere. Usually they live in the plugins/ folder of the site so that should not be a problem. But plugins often bring up python dependencies.
For example, a plugin to use commonmark will require the commonmark python package. I can't install it in the snap, since it's readonly. So, no idea how to handle this. Should I ship pip? Add something to PYTHONPATH and install there?
2) It uses python's webbrowser module. How that works under the hood, I don't know.
3) I don't know.
4) I don't know. Literally, anything can be used this way.
In any case, I am happy to keep pushing a semi-functional snap as it currently exists. I stopped supporting usage of the debian package because it was never updated, and as mentioned running from source is not exactly trivial at times.
So, if this doesn't fix the issues I guess I have options:
1) Stop caring about it, users use sources.
2) Look for other ways to get this to work that are not snaps.
Sadly, time I can spend on this is basically no time at all. Feel free to close the issue if classic is not the right solution.