Yes, the FAB Defense Gradus Rubberized Ergonomic AR Pistol Grip will fit and function as intended with your FX-9 because the FX-9 is designed to accept all AR15 style pistol grips.
The manufacturer doesn't list the part number for the FAB Defense Gradus Rubberized Ergonomic AR Pistol Grip plug. I recommend contacting them for that information.
No, the A2 grip plug will not fit.
This is a link to screw you will need for installation https://www.opticsplanet.com/advanced-technology-international-ati-ar-15-pistol-grip-screw-and-star-lock-wash.html
You should not need to make a hole to install this, it should already have the hole at the top for the screw. This grip screw should work for you: https://www.opticsplanet.com/advanced-technology-international-ati-ar-15-pistol-grip-screw-and-star-lock-wash.html
No, this does not have a storage compartment.
i didnt hsve a issue. If anything trim the spring little by little till you get a good tension on the safty selector.