So I have been trying to observe whats going on with plugins. This is what I found.
If I install picard with "including plugins" option, only these plugins will show as installed and are able to be ticked:
Other plugins don't work at all, as described in above posts.
Also with this option, all the plugins are located in C:\Program Files (x86)\MusicBrainz Picard\plugins (including those that don't work).
However, there is also a folder C:\Users\UserName\AppData\Roaming\MusicBrainz\Picard\plugins. If I copy plugin file into this folder and it is one of the plugins that is not working by defaultt (as in, those that are NOT stated as "Installed" on the screenshots), it will work. For example, I copied release_type.py in this folder, clicked on it and it works. But if I do the same and copy it to C:\Program Files (x86)\MusicBrainz Picard\plugins, it won't do anything.
So all in all, it seems like certain plugins will only work when installed to AppData location? Why is this?