as to what pin to use, i don't know,

IF pin 6(pwm_2) is used by something else, what is using it? if it isn't shared by anything potentially important you might still be able to use it you might not...

you also have pwm_1 which is pin5, I don't know if the servo need 5 or 12 volts though, i think 5... which means you might have to get creative with connectors if 5v is needed by the servo.

as to the firmware i suggest watching the youtube videos by zennmasterM for setting up ABL, they are as close to a step by step guide you can get, for software and procedure to get offesets etc setup correctly.

As to when you home all axis or home Z it will raise the z axis extend the probe and then lower Z until it hits the endstop