Just destroyed E90. It has 125 x 40 mm keyboard in 5 rows without shortcuts. I've tried to see key-matrix but it was insanely high-tech-space-age. Instead of buttons it had small strips of metal hopping on circuit.
E90's mat fit's perfectly and leaves some space for grip and allow's even to reveal camera... at least that's how I figured it out: You lower your keyboard just to take a photo.

I loved that keyboard

Now I'm wiring it for third time with directional keys at the spot where E90 has them. Instead of a huge enter-key, I'll set enter to the middle of the directional keys. I squeeze 5 rows in to 4.

Are you Dirkvl still using N900 -mat. I recommend this one.

Diode in each key (in combinations) that causes ghosting solve's the problem. Are you still sure about pull-up-resistors since manual says both: doesn't need and does. Or doesn't need them somewhere else than SDA and SCL.