I believe the ruling was in the latter part of the 1915. The letter is dated March 1914. The reason usually given for the ruling was because he owned a sporting goods store, but i wonder if this ad (or others like it, if there are any) had something to do with it. I think had he not gone into the sporting goods business or had got out of the business he would have retained his amateur status. For example John G. Anderson (one of the more prominent golf writers of the time) was employed by Wanamaker's as a wholesale buyer and seller of golf equipment, and he gave up his position after the ruling in order to retain his amateur status. (Ironically, his byline almost always noted he was runner up in the 1915 Amateur, which seems a pretty direct effort to capitalize on his status as a golfer to me, but writers were not excluded under the 1915 ruling.)
As for how long he was a "professional," I don't think the he ever considered himself such. It was written that he felt that, while the USGA could bar him from the Amateur Championship, the USGA could not make him a "professional", and that his position was that if he ever won any prize money he would not accept the money.. (Not sure if this ever happened or not.) I think he was reinstated in 1919.
My guess is that he was never directly paid for the ad, but wrote it as part of his employment for Wright and Ditson. But then that more questions.