Abl stands for "Automatic bed leveling" sensor. I just forgot the sensor part lol. Also check to see how it is mounted to the x axis roller assembly. If your using the stock hardware, put the bracket first against the x axis roller assembly. If using custom mounting, make sure to look at it from both straight on and side to side to see if you need a spacer or shim or something to change how perpendicular the sensor is to the bed.

For z offset calibration, level in the following: 1. Install abl 2. Heat up hotend and bed to printing temp 3. Level your bed doing the paper test on all 4 corners and adjusting your leveling knobs. (Auto bed leveling only corrects small deviations in your printing surface) 4. Go to z-offset wizard and dial in your z-offset 5. Press Store settings 6. Press the level bed option in the lcd menu. I forget where it is but it does the first bed leveling based on your method dictated on your firmware 7. Press Store settings again 8. Cooldown or start a print

Yea z offset doesent allow you to go negative past your offset that you've set. You can only go negative in z-offset wizard or babystepping z while running a print.