Can ECC RAM fix instability caused by undervolting?

Not really. ECC corrects memory errors but doesn’t stop them from happening, so it just hides symptoms of an unstable undervolt. An unstable memory controller can still cause hangs, crashes, or file system corruption before ECC acts. Fix the voltage instead—raise SOC voltage or back off the undervolt. Don’t use ECC as a crutch for a too-aggressive undervolt.