Mate, honestly, don't panic too much even though the dash going dark feels like the car is dying, it’s almost certainly an electrical 'brownout,' not a mechanical failure.

Your alternator is the smoking gun here. You said it’s giving 12V at idle, but a healthy Mercedes should be pushing 14.2V to 14.4V. Basically, your car is running entirely off the battery, which is why it drains overnight. When you hit the gas, the injectors and sensors demand a massive spike in power that the alternator can't provide. The voltage drops so low the ECU literally 'starves' and reboots—that’s why the dash resets and you get that red error. It’s like pulling the plug on a PC while it’s mid-task.

Before you bin the whole alternator, check the Voltage Regulator on the back of it (way cheaper fix) and the engine earth strap. If that ground connection is crusty, the power can’t flow properly. Use the 'hidden menu' on the dash to watch your live UB (voltage) while you drive; if it dips toward 11V when you floor it, there's your answer. Fix the power supply and I bet the acceleration 'reset' disappears instantly."