IMO, it's not that a free period is deadly; it's the courses one takes and whether the free period could be put to better use.
IMO, the OP's schedule is a bit "soft" for USNA purposes. Pre-calc vs. calculus and not AP. Government/Economics vs. history (understanding the OP's course may be state mandated) or an advanced language. Physics is not AP. ROTC is not a "core course," and thus doesn't carry the same weight from USNA's perspective as a course in math, science, English, history, etc.
Given the above, I would recommend the OP take another course vs. the free period. That said, it would be better to get all As in the courses listed with the free period than to add a course and drop to Bs and Cs.
If the OP's overall record is very strong across the board, the free period probably won't matter. However, if the OP ends up "on the bubble" for an appointment, as many candidates do, the current plan could hurt a bit as compared to other applicants who are taking 5 "tough" courses.