I don't give up 'my will to make it work' fast but it seems to me that it really can't work....
I don't understand it. Even when I wait 2 seconds (!)with the interrupt disabled (!) before reading the keys the INT still fires twice. Even with no I2C read actions on the expander. I checked if the 89C668 has the same adresses for interrupts etc. There is a difference in some PCA registers !
Too bad my PCB is already etched.

I think I'll go for scanning the keys with 3 HEF4021's. I saw a nice piece of code in de Code libary for scanning keys.

Goes to prove that a bad experience is an experience as well