"I am not a developer but this can be my contribution to q2a" -> That is exactly the attitude most users should have :) Answering your questions:
1. The trick here is to change the values in those 3 files. Firstly, metadata.json files will be looked for (they're already there). If not found the qa-plugin.php file in plugins will be searched. Now, if you change the name (or any other value) from metadata.json from X to Y you should see Y, regardless of the value in qa-plugin.php, which should be X. If you remove metadata.json file, as it is not found, you should see X as it is taking it from a the qa-plugin.php file (old way). Same thing with qa-styles.css. Just change the values and remove metadata.json files
3. I see v1.3 so you have changed the metadata.json version key properly. But have you changed the "update" key to the link I've provided? I noticed core plugins/theme do not have "by default" the update key set so I didn't add it to use the same logic. That's why you have to manually add it. Make sure it is there and try again.
6. This one is actually the easiest to test :) In the default installation you'll find English UK (the one you've set in the screenshot). Browse to the qa-lang/en-GB directory. In there you'll find the metadata.json file. The text should be the one in the combo. Does changing it also change the combo? What if the whole "display_line" is not there and there is only curly braces? What if the file is empty? What if the file is not there? Does changing the directory name from en-GB into es-AR make any difference at all? Just some ideas.
Thanks for taking the time to test this :)