Is it permissible to have an optional reprocessing step, for example a re-slurry, as part of the manufacturing instructions, where the decision on whether or not to reprocess would be based upon an evaluation by testing of the purity of the in-process material?

This is a common practice. Many API processes have an in-process check (for purity, etc.) before
continuing or initiating a re-crystallization or a reprocessing step. This check enables you to
determine whether or not you have bet specifications to proceed. It’s built into the batch record;
it’s part of the established process. It’s triggered if the purity in-process specification is not met. If
met, you proceed to the next step.