The error *implies* that the application is already stopped and the webUI just didn't update for some reason. IIRC (and I could be wrong), the webUI in 6.3 and earlier didn't always refresh itself after a stop.
It should actually always take less than 10 seconds. The standard command issued with unRaid to stop an application (you can do this yourself) is:
docker stop NAMEOFCONTAINER
It tries to perform a graceful shutdown of the application, and if it doesn't gracefully exit, then the docker system will forcibly shut off the application after 10 seconds
Different philosophies on how to build an application. Generally unless there's a very good reason to pick something else, you're going to go with linuxserver.io (if you want the applications to update themselves to the latest version every Friday), or binhex versions (if you prefer for binhex to manually release an updated version when he does it).
Post in the support thread for qBittorent. Link you can get via the apps tab
Like everything else, without diagnostics, its very hard to tell.
IMHO, unRaid is 100% rock solid. Especially on the docker side of things. TBH, I would recommend upgrading to 6.4 as that is the version of unRaid that most of the docker experts themselves are running (including myself) 100% trouble free. But, I also ran 6.3.5 (and every 6.x release) troublefree
Once again diagnostics, but there is one issue common to most new users - the docker image filling up due to misconfiguration of the applications. And that "issue" causes all sorts of strange trouble. But it is not the fault of unRaid or Docker. Its a pure and simple misconfiguration.
Yes
Depends. If the problem was with the docker image, then yes deleting the image is basically a clean install of every application. If the problem is with the appdata / metadata for the apps themselves, then this will not solve anything, and you would also have to delete the appdata in addition to the image.