Hello,
There is a weird behaviour with the KAR format for addon on the marketplace (following a certain naming convention makes unexpected result, autoremoval and double display ?).
I’m not 100% sure, but I may have fixed it with the latest version I just uploaded this morning.
At least it doesn’t show up twice in my system anymore (I’m on 4.1.0, so take it with a grain of salt).
You can try to remove and reinstall it from the marketplace, and tell me if my new version does the trick.
Note : If you have some other issues, don’t hesitate to use the “main” marketplace signal thread, I receive notifications and can answer faster.
NB : I cannot answer to the other questions, I’m also at lost when trying to figure out where markeplace addons are stored 