Yes, it was the only one to take part, so? They still built four, had another class of four +1 building (never bother to finish them, they weren’t needed at the time), and there was also the Dictator. That’s nine ocean going Monitors, 20 coastal Monitors and dozens of other river & harbor monitors.
Yes, it was towed, as it couldn’t carry enuf coal. It was still capable of sailing in high seas. "Departing St. John’s on 5 June, the three ships crossed the Atlantic in less than 11 days. Fox described the first ocean crossing of an ironclad monitor as “a pleasant trip.” During much of the voyage she was towed by Augusta “as a matter of convenience and precaution rather than necessity.”…In company with Augusta, Miantonomoh departed Gibraltar on 15 May 1867. Steaming via the Canary and Cape Verde. Islands, Caribbean ports and the Bahamas, she anchored off League Island, Philadelphia, 22 July, thus completing a cruise of more than 17,700 miles.
The reaction of the British? She returned to the English coast on 7 July and a week later received visitors including British royalty, government officials, and members of the press, all of whom viewed her with wonderment and amazement. Her departure in naval design caused considerable comment in the English press, and the Times exclaimed: “The wolf is in our fold; the whole flock at its mercy.”
They laid down their first turreted battleship as soon as they heard about it. It wasnt launched until 1869, and was rather similar except it still had sails and a high board. The Monarch* “Having determined that Monarch would carry her main artillery in turrets, the Board of Admiralty then stipulated that, as she was destined for overseas service, and steam engines were not at that time wholly reliable, she must carry a full ship-rig and be fitted with a forecastle. Reed objected to this concept, which had the effect of totally preventing the main artillery from firing on any other angle than on the port and starboard beams. He was overruled, and is reported to have taken little pride in the resulting ship. He himself wrote, in 1869 “no satisfactorily designed turret ship has yet been built, or even laid down…the middle of the upper deck of a full-rigged ship is not a very eligible place for fighting large guns”.”
*
It wasnt until 1873 that the British navy had anything that could match the US Navy’s monitors.
Note the rest of your quote “In 1865/6 she went to San Francisco, **via the Strait of Magellan **and although three ships were in company, she was not towed.”
Even the best sea-keeping ships of the time were in danger going thru the Strait of Magellan.