This is Sig's statement about changing MPX triggers....
MPX Trigger Compatibility
The SIG MPX has a fire control group specifically designed to accommodate the high impulse of the MPX operating system. The MPX Trigger Bridge (indicated below by arrow) is in place to reduce wear and extend the life of the factory installed fire control components. Removal of the Trigger Bridge is NOT recommended and will void the warranty of the firearm. While the SIG MPX fire control components are similar to the AR15 platform, and aftermarket fire control components may fit, installation is NOT recommended in the SIG MPX. Doing so may result in damage to the MPX proprietary fire control components or the aftermarket components you installed.
Are you saying Sig customer service told you it was Ok to put in an AR trigger?
Northlight