I found a solution that does the trick for me. This removes the middle column with word suggestions and the paste button from the landscape split view (which can be activated in Settings->Textentry->split keyboard), leaving you with a landscape keyboard with big keys that covers less than half of the screen. To use the pastebutton, you will have to switch to portrait mode. For me, this is perfect, because I personally don't use copy-pasting that often.

So here you go:

• Become root (devel-su).
• Open the file /usr/share/maliit/plugins/com/jolla/KeyboardGeometry.qml, consider making a backup file.
• Quite in the center of the file, change
  
  Code:

  property int middleBarWidth: keyboardWidthLandscape / 4

  to
  
  Code:

  property int middleBarWidth: 0

• restart mailiit-server by
  Code:

  killall maliit-server

• Done! :-)

- tested with SailfishOS 2.0.0.10 (Saimaa) -

Known issues:

• The Opera Mobile Android app seems to be special regarding the keyboard. It is the only Android app - or app at all - that I tested still displaying the paste button. You can't tap on it, so it's no actual problem, but it is sufficiently ugly :-) Possible workaround: Switch to portrait mode, hold the paste button and delete the clipboard contents. Now the icon is no longer shown in landscape mode until you put something to the clipboard again.