How do I enable a keyboard if I can't open Settings?
Yes you can enable an installed keyboard from the Terminal emulator (ime enable/set xxx), but you still would have to type something and you said you don't have a keyboard...
Get a hand on an OTG adapter, this way you can directly plug a physical USB keyboard and type what you need.
You could also install a keyboard like SwiftKey (from Google Play desktop). You should be able to open its app by clicking in the notification tray then it will help open the part of the setting screen to enable this keyboard (if this part doesn't crash).