I’m sorry for not being clearer. No, Cosmo (as far as I can tell, looking at published lists and online data) was not one of the individual duelers, or in the team fencing competition, at the 1908 Olympics. This was supposedly due to an "administrative" reason that remains unclear. He was involved in the games as captain of the reserves, as a coordinator, and also as a "representative" of the home team in some official capacity, though to what extent I’m not sure. I believe he was a social "host" of sorts, probably appointed by his friend, Lord Desborough, president of the British Olympic Association, but I haven’t proved this yet. Whatever Cosmo was doing by 1908, it was in an organizational or diplomatic capacity which, depending on how you look at it, was more (or less!) important than being an athletic participant.
The confusion has been (for me at least) that some 1908 news reports referred to Cosmo as a "star member" of the British fencing team. Was this a mix-up of biographical information from the 1906 Athens games? Also photos of Cosmo were published at the time of the London games, showing him holding his saber, etc. In the group photos of him, however, he’s standing with men I’ve been able to identify as members of the reserve team. I suppose it’s possible Cosmo was pulled from the reserves at the last minute to replace an absentee member in the individual competitions, but (as you point out) he must have placed below 8th place.
If someone can definitely prove Cosmo was a dueler in the 1908 Olympics, I’ll be glad to have it confirmed but nothing I’ve seen so far shows he participated beyond the status of a high-profile auxiliary member. I’m still looking for substantiation that he was one of the Olympic committee’s "hosts" in 1908; it’s also quite possible he was involved on the finance end of the games, as he was an investor in a number of businesses besides his wife’s.