It doesn't lock the power key unfortunately, that's the intended behaviour. I have found that the longer you have it, you get into the habit of avoiding switching it off.

No, there's no way to do that. If you're worried about the battery going off, plug it in before going to bed. Or put it into 'Airplane Mode', that'll shut down almost everything that could drain the battery, it would have to be almost dead to run-out overnight

 

you better optimize your phone settings for a longer battery backup. Check this for battery backup and know about how to condition your battery here.