On the firing pin safety issue, it was in part a fix for a non-problem. The bolt carrier is light enough that if you drop the bolt on a manually chambered round (one not stripped out of the magazine, but dropped manually into the chamber) there is a chance it could slam fire. So Sig redesigned the carrier, added a firing pin retaining spring, lightened the firing pin, and added a hammer-activated firing pin stop. The firing pin stop is not compatible with lightened or match hammers. The stock MCX has a lousy trigger, and desperately needs an aftermarket trigger.
Since all this was done to fix a non-problem, the first solution is to not drop the bolt on a manually chambered round. The next would be to install the upgraded bolt carrier, but remove the firing pin stop while retaining the firing pin spring. This will allow the MCX to use better triggers+hammers than stock. The new firing pin stop is also a device that might make the rifle less reliable or prone to malfunction. I'm not convinced that it is reliable.
YMMV.