Therefore when the keypad scan commences, a row will be low.
that will be all rows OR all columns plus one of the other

When a button is pressed, P1 != 0xFF
do you mean all rows and one column or all columns and one row

I set each column high in turn, and check to see if P1 == 0xFF after each column goes high.
"each column goes high", those with open switches will be high as well.

It may be that you have a "new and revolutionary" way of doing this, but I do not see it. I think you misunderstand that you can not "set an input high" it will be high or forced low.

Erik