Hmm, how would one use this for falling edge trigger?
Several possibilities there.

If using Option #1, the 4th NAND gate is available and can be used to invert the incoming signal. Selection of Normal/Inverted modes would then be with a Jumper or SPDT switch.

If you wanted it to be selectable by the PIC, you could add a 74HC86 quad XOR. With the extra gates, you could put an XOR on the output too if you wanted the option to invert it from the PIC.
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