Don't know why all the troubles with getting the data but I do see where you are trying to grab the data on the falling edge E (enable pin).
I went and looked at what I've written for the avrs (I usually write in assembly and have just gotten into the arduino) and I also looked at the arduino library for the lcd's . The enable pin is usually held low when not writting to it. The data is placed on the bus then the enable is brought high for a brief period then it is brought low to write the data to the display.
If it were me I would enable the interrupt on the RISING edge of the enable pin.