Please try invoking the switch to greek from the KM status menu
Where did you get the Greek keyboard definition file from?
Is this a standard Apple key file definition?
It might be worth trying a "different supplier/source" for your Greek key definitions
Apart from finding your original post, I also found these, which might be relevant,
There is also a hint on stackoverflow that you may need a command +space or similar to complete the switch, but I think yours is failing before that point.
Finally there may be an alternative way using a shell or applescript.