2008 Mercury Milan (Ford Fusion) Gear Shift Ratio Code: Is Transmission Rebuild the Only Solution?

My 2008 Mercury Milan (also known as Ford Fusion) started exhibiting gear slipping and rough idling. The dashboard wrench warning light came on, but no OBDII diagnostic codes were initially detected. After checking the owner's manual, I found a reference to potential throttle body issues. Researching similar symptoms led me to believe the throttle body sensor was faulty—however, since my car has an integrated sensor, I had to replace the entire throttle body assembly. I attempted a DIY installation but failed to notice that a small hose connected to the throttle body was filled with antifreeze. During reassembly, some fluid splashed, so I quickly reverted to the original throttle body. After multiple restart attempts, I followed proper system relearn procedures as recommended in the manual. The car started successfully, but then triggered an OBDII code indicating a lean mixture and activated the check engine light—likely due to repeated restarts. I took the vehicle to a mechanic who replaced the throttle body without first confirming that it was indeed the root cause of the symptoms. He installed the new unit and claimed the 'Gear Shift Ratio' code (specifically related to 2nd gear) was normal, stating it's just a running hood message and not indicative of any serious issue. However, within two blocks of driving, the wrench light returned, and the car began shifting improperly—now slipping in gears and refusing to shift into high gear. I then visited a transmission specialist who confirmed the 'Gear Shift Ratio' code and suggested a full transmission rebuild. Upon inspection, the transmission fluid level was found to be higher than normal on the dipstick. This raises concerns: could replacing the throttle body have triggered or worsened transmission-related issues? Based on research, possible causes for the Gear Shift Ratio error in 2nd gear include: low transmission fluid, contaminated fluid, a faulty Powertrain Control Module (PCM), or blocked internal fluid passages. I'm seeking input from experienced mechanics: is a full transmission rebuild truly the only viable solution for this issue? Are there any alternative diagnostics or fixes that could resolve this without costly repairs?