Checking the Treat hardware deadkeys as plain keys option does help, but there are still some very strange things going on when using Keyman with the Phil Unicode keyboard on a Norwegian physical keyboard.
In Word 2010, if AltGr + a dead key is pressed at the very beginning of the document, it has the effect of locking the Ctrl key, with all sorts of strange things happening when subsequent keys are pressed. The only way to get out of that state is to press the Ctrl key. If AltGr + a dead key is not part of the first character in the document, everything works normally. Everything also works normally if the Ctrl key is pressed while pressing AltGr + a dead key for the first character in the document.
In Paratext 8, any time the AltGr key + a dead key is pressed, the whole chapter is highlighted. Also here it has the effect of locking the Ctrl key and it is necessary to press the Ctrl key to get out of this state. And also here, if pressing the Ctrl key in combination with AltGr + a dead key, everything will work normally.
It looks as if things work the same way in Notepad as in Paratext 8.