Let me sum this up. First, ALL ICS updates install on top of 5.9.905. So when the OTA comes out, you will need to be on 905 in order to take the OTA. This can NOT be done while on a leak.
FXZ files allow you to restore your phone back to 905 minty factory fresh. This is done by using RSD Lite to apply the FXZ. Your data and apps will be wiped. This will then allow you to take the OTA update. This can ONLY be done if you are on 6.7.232 or LOWER.
If you are on a 6.7.235 or higher leak, then things are WAYYYY more complicated. Those that have ignored the warnings about those builds will NOT be able to go back to 905. That would be what the warnings were all about. For you, the ONLY solution is to move forward. This is not as "simple" as installing the update since it has to install over 905. You need the boot img of the update you're wanting to install, then you need to flash the system img, preinstall img, webtop from 905, and the boot img from the new update. Then you reboot into recovery and install the update. How awesome.