Power dissipation of a current limiting resistor (connected to an input pin of uC.)
First question     where is that trickle of current going?    It's leakage current through the diode.   Got a datasheet?   Its probably specified

Second question ( actually labeled #1).    The max 3mA is when the port is an OUTPUT
You seem to be wanting an INPUT yes?

The current would be close to zero.  Except if you have am internal pull resistor active.   In other words... "What is the input impedance of your I/O port?