"If you were to compare starting SCHWA characters to AD&D1e characters, what level would they transfer to? 3rd? 2nd?"
Short answer: I wouldn't compare them to AD&D characters. Mostly because of the math.
In any iteration of D&D, even those games that are only nominally D&D, only a single die is used to determine success or failure, the d20. Bonuses and penalties are added to the result of the roll and the range of possible results is only 20 wide. It may be 1-20 or 17-36, but always only 20 wide.