Just a couple quick notes....

Merchant ships being unarmed is a recent anomaly, historically. Until the 20th century merchantmen were routinely armed to deal with (drum roll, please) Pirates. Piracy had largely though not completely) been eradicated by the Royal Navy and the US Navy, removing the need to arm merchant ships. Even into the early 20th century merchantmen normally had a small arms locker to deal with issues.

If Letters of Marque have been granted by Congress anytime in the last 200 years, I'm unaware of it. Handy way to deal with unspoken wars (basically a letter of marque is legalized piracy), but I don't believe we have used them in the post revolutionary period.