amendment of indictment was permissible because indictment was facially valid and amendment did not add any previously omitted essential element of the offense charged, resulting in a substantial alteration; proof of sexual activity by a substitute parent or custodian does not require that person who assumed position of a parent reside in the home with the minor victim; consistent distinction between predicate acts for two counts of sexual activity by a substitute parent in jury instructions and verdict sheets demonstrated that jury verdicts were unanimous, though distinguishing between the predicate acts in the verdict sheets was not required; indictment charging multiple counts of a single crime based on distinct predicate acts was not multiplicious; distinct sexual acts occurring during the same sexual encounter can provide the basis for multiple charges of sexual activity by a substitute parent or custodian; trial court did not abuse its discretion by imposing consecutive rather than concurrent sentences