Help me understand this one thing for now. I think we both admit that not all adults who are baptized receive the thing signified, since some do not have true faith. Further, believing adults already possess the thing signified in some sense before they are baptized.

This is viewed as fine and not "separating sign and thing signified," which has been your concern in the past. But if signum and res can be separated in the case of adults, so that res comes before signum (for believers), and signum can be had without the res (for unbelievers)...

...what's the problem with affirming the same holds in the case of infants--i.e., elect infants receive res before signum, and non-elect infants receive signum but not res?