Ok I think found the answer to my chronology conundrum. I found that it was D. Martin Dakin's Sherlockian book where I had read about the October date for the Orontes troopship in A Study in Scarlet. Strange that the Annotated Sherlock Holmes didn't make a note about it at all. Dakin describes the previous research on the ship by Percy Metcalfe, and he cited a "Sherlock Holmes Journal" article as the source. But of course, that was from decades ago and I despaired of being able to buy an obscure SHJ article anywhere online. They do sometimes show up on Ebay, but the "Baker Street Journal" issues are fare more widely available.
Just when I was despairing of ever locating it, I found a 2 volume book called The Grand Game: a celebration of Sherlockian Scholarship edited by Laurie R. King and Leslie Klinger. It basically gathers together lots of different Sherlockian essays from several decades, and I was able to see Metcalfe's article listed in the table of contents. The 2011 book was published by the Baker Street Irregulars, but their website listed it as sold out. I searched some rare bookshop sites trying to find a secondhand copy to buy, but it was nowhere. I began to despair again, until I found the book on OpenLibrary, where I could create an account and "borrow" the book so I could read the digital copy. Hooray!
Percy Metcalfe's 1959 article began on pg 83, and he says that he looked for the Orontes ship in the Naval and Military Intelligence column of the Times from July 1880 to December 1881. He found that the Orontes was pressed into "special service" no doubt because of the huge defeat in Maiwand that month, and was sent with some troops for Afghanistan and India on August 3rd, 1880. (So even though its regular routes were the West Indies and South Africa, the ship could be diverted when needed.) The ship arrived in Bombay on September 1st, delivering the troops, then it waited a couple of months.
On October 31st, she left Bombay for Portsmouth... [arriving] on Friday afternoon, November 26, bringing home the troops from Afghanistan, including 18 invalids.
So Metcalfe is theorizing that's when Watson went home to England. For months afterward, the Orontes ship was back to transporting troops to and from the West Indies and South Africa, its usual routes, so it didn't sail that Bombay-Portsmouth route again. As he said, he had checked all the dates through December 1881 to make sure there wasn't another special trip.
The only problem left is how Watson got out that early, when the rest of the 66th regiment was besieged in Kandahar and not able to leave India until January 1881. Metcalfe addresses that issue in the rest of his article. Citing some contemporary military dispatches about the Battle of Maiwand, he notes that "the 66th Foot (the Berkshires) were indeed with General Burrows on that fatal day (July 27th, 1880), except for two companies under Major Ready which were on detachment at Ghilzai."
Metcalfe is not saying Watson lied. He's saying that Watson served at Maiwand and got wounded in the battle, but then Murray threw him on a packhorse and rode out to Khelat-i-Ghilzai to join the other part of the regiment, thinking he would be safe there. Watson was too injured and faint to know where exactly he was. Eventually the wounded people were taken to Kabul, not Kandahar, then later they were sent to Peshawar hospital, while the rest of the survivors retreated to the garrison at Kandahar. It's suggested that these soldiers had enough to do defending the fort, and they didn't want to be responsible for wounded troops too. I mean, there were sick and wounded people at Kandahar too, but they could have become so over the month-long siege.
So apparently that's how Watson avoided the siege at Kandahar, came down with enteric fever at Peshawar, but eventually recovered enough to be sent home on the Orontes in October 1880. Well, I mean if you want to keep A Study in Scarlet in 1881; other Sherlockians prefer 1882 or later, and have their own justifications for that. I guess I could have gone either way on it except that I didn't want Watson to be mistaken about the ship name.
