Thanks for that, Bill.
My sources on the date are the newspapers, and they will have syndicated the mails that came out from Bombay which stated that the Malabar arrived at Bombay on Friday 13 Nov - this was reported on Sat 14th Nov, which would seem to discount the Sun 15th Nov date. In many pieces it went along with the news that the Malabar's captain, Capt. Nott, had died on board during the journey (Oct 26 at Ismailia).
I guess we can't be sure of Gretton's sources for his book - it may be that the ship was expected on the 15th but arrived early - arrival estimates weren't always accurate - and he could have found that date. Or he may have misread a document, mistranscribed it in his notes, or copied a mistake.
Or he cold be right, and the newspapers wrong. Sometimes the dates in newspapers are a little off, but the reporting for the Malabar arriving at Bombay seems very unambiguous (eg. it "arrived yesterday", rather than it being "expected" today or tomorrow, etc).
I think I'd go with the newspapers on this one, but keep an eye out for other first-hand sources to confirm.