1. The plugin has not been updated for the new version of JM, so is not usable right now.
2. The new version of Electrum fixes the bug that had to be manually fixed in those install instructions, but for now this is irrelevant because of 1.
3. I am overwhelmed with working on Joinmarket itself, so I have no time to update this, I expect it will be a couple of months before this is re-addressed, and when it is, I'm not sure in what way. If someone else wants to do more work on it I'd be happy to answer questions.
4. I don't know about wallet file format changes; the plugin as-was was only supporting "Standard" wallets fwiw.