Understanding MCU Reset Pin working
1) Yes it is bidirectional

2) It is not specifically mentioned but it is otherwise obvious from the electrical characterisrics for example, it is an input than can drive low.

3) Why would reset pulse length matter? But an oscilloscope or logic analyser will display the reset length.

4) WFRST can be searched from the datasheet, it is maximum pulse length filtered (ignored) by reset pin. Minimum current is in elelectrical characteristics (Max 150 microamps), the voltage must be pulled below 0.35*Vdd.