Hi Richard,
Thanks for the update. I couldn’t find your support topic in the respective plugins support forums although feel free to share.
2) Sorry, I did NOT find the extension on your github page. How can I get it?
The extension can be installed via opening the “Code” page and then “Code > Download Zip”. You can then upload this zip file as a regular WordPress plugin. Once activated your navigation menu will work in AMP as it does in your non AMP URLs.
3) Is there another better structured data plugin that you can recommend that is fully compatible with your AMP plugin?
There may be no need to change if your current one works fine using our AMP plugin, which should be the case from previous tests. If you do prefer to change you’ll find others, including Schema within the WordPress plugin repository. We’ll also perform some additional checks on our side to ensure your current plugin does work in AMP while keeping your updated here.
4) My articles do pass the Google Rich Snippets Test so maybe things are OK despite the weird software behavior?`
That’s great that they pass the Rich Snippets test. I would imagine you’re correct and it has no impact. I’m unsure if an individual content type added for structured data is needed however I will check this further.