While there might be some problem with the repo keys, that will have to get sorted, I can tell you some more about your other questions.
You should be able to directly upgrade to the latest version, as any 9.1, 9.0.9 and version 8.2.11 can directly upgrade to 10.2.1:
So, aside from possible repository problems, you should just be able to set your repository to the one documented here:
https://download.owncloud.org/download/repositories/stable/owncloud/index.html
And then upgrade, but please take a backup before you actually go ahead.