> hidden WiFi?.
If your wifi works on nickel it should work on KOReader too. If you're trying to add a new network then do it in nickel and it should be available the next time you run ko.
If you're attempting a harder setup where you want to choose one AP from the list of available networks, graphically from KO, then the best alternative right now is to unhide your network
> can I configure Koreader to start automatically
That's a bootstrapping issue. You can't change a program itself to start automatically. You'll need to change the firmware where KO runs. It is possible but
not recommended unless you know what you're doing.
> open directly last document?
On the filemanager menu you can change the behaviour at start. You can choose between the filemanager, last document opened or history view. That sadly doesn't fix the bootstrap issue.