I have a mod that's causing a critical launch error and Reforger will not launch at all.

There needs to be a command line parameter from the game in the Steam library, General, and Launch Options that will force mods to be turned off or unloaded so the game can launch. One would simply load each (one at a time) of their downloaded mods to see which one is causing the problem and uninstall or move it to inactive until it can be updated and fixed.