How would the criminal justice system be different today if the Founding Fathers had allowed state courts to enforce federal laws and had not created a separate system of federal courts?

Answer 1

Without separate systems for State Courts and Federal Courts, the boundary between States and Federal government would be blurred. This would dilute the powers of both States and Federal, create controversies, and power struggles to find an equilibrium between States' and Federal to enact and enforce laws.

For example, if the Federal level made a law that one of the 50 states found unreasonable, the State could try to avoid charging under that law. 22 more States might join that argument, so the issue could split the US States both for and against Federal legislation. No one would get anything done except trying to clarify these disputes.