You are welcome.
Colonel Hugh Seymour married Charlotte daughter of the 1st Marquess of Cholmondeley 18 May 1818. He died 2 December 1821. Hugh was 2nd son of Lord Hugh Seymour and brother of Horace B. Seymour.
Presumably the memoir was written after the facts, so, it is possible that the reference to Major was the rank held when the subject died and not necessarily the rank held in 1814 or 1815? If you have the approximate year, you could check the yearly army lists for the names of casualties and alterations since printing to see if any Seymours show up. Half-pay officers are included.