all pressed keys to be sensed, please let me know.
1) you can not detect "all keys" without including diodes. Without diodes, you can only detect "any key" i.e. multiple keys pressed is not detectable with certainty.
2) there really is no reason to use a keypress interrupt since you have to scan the keys anyhow (row select) and for the purpose of debouncing a timer (interrupt) need to be involved anyhow. Using a keypress interrupt will, typically be a heavy load because you get a whole lot of them when the key bounces. Using the PCF8574 is, with the exception of the IIC communication, not diffrent from using a regular '51 port. search on "debounce" and solutions will come up.

Erik