Plausibility check: I have added a "check" button in a transaction that performs additional plausibility checks for the data entered in an InputScript "plausibility_checks.txt". If the InputScript does not find an error, I would like to continue in the transaction without an "Enter" being performed, since SAP will display an error message if the user has not yet filled in all required SAP fields. How can I achieve this?

Your InputScript probably doesn't contain any "Screen" commands, only the plausibility checks. In this case use a "Return" statement at the end of the InputScript; this avoids the processing of the SAP application when the InputScript is finished.