logging in as admin, you should have a "cloud" icon containing the public images folder, where you can upload images to be used later e.g. for mailcards.
Here you can upload the login.png file and name it exactly like that to have it as the new login page.
The problem you're having (logging out users) shouldn't happen.
If it does it means that or the websocket or other continuous requests are being blocked, not allowing the session to keep alive.
You may open the debug window of Chrome (F12) / Network pane, leave WT5 there without working on it for some time, and take a look at the network requests, if any becomes red that may help investigate what's happening.
Nextcloud will be supported on WT5 shortly.
At the moment, it supports its own local resources, url connected resources (smb, sftp etc) and public clouds such as Google and DropBox.