I wasn’t able to edit a hunk (like the e key in git add -p) which I use a lot to split debug statements from real work

I don’t think the builtin diff editor can do this, but you can set a different diff editor than the builtin one: https://github.com/martinvonz/jj/blob/main/docs/config.md#editing-diffs

edit: but wait, debug statements? Are they mixed in on the same line as the real code? The builtin diff editor can pick changes per line.

I found no way to show the original diff

jj evolog to show how a single change evolved including the previous commit that didn’t have the conflict yet, if that’s what you mean.

jj undo did not worked (I have not been able to undo the jj squash that introduced the conflict

If you did something afterwards, the operation you undo will no longer be the squash. Look at jj op log to see which one is the correct one to undo.