I spent many summers at Lake George from the early '60s until just last year. While I have nothing to prove 1957 was not when the line was abandoned, I have distinct memories of the crossing of Rt. 149 south of Lake George Village still being intact complete with crossing signals. Since I was born in 1958, that couldn't have been earlier than 1961 for me to have any memory of it. But I have no memories of ever seeing a train so perhaps it was abandoned but nothing had been removed yet.
I worked the summer of 1974 for the LG Steamboat Company. At that time, the Steamboat Company owned the station (called the Station Restaurant at the time) so I had a chance to explore a little. I remember then in the area between the station building and the miniature golf course just to the east that the platforms and trackways were still present and visible from the street.
At one time, the Steamboat Company was a D&H subsidiary (you could take a train to Lake George, steamboat to Baldwin (south end of the Ticonderoga Branch), and then train again) so I think their ownership of the station goes all the way back to when the D&H sold the Steamboat Company. According to a book I have on the history of the Steamboat Company, that was in 1939 so it's not clear how that really worked unless the D&H retained a right to use the station as part of the sale. From pictures in the book, the current road between the station and the steamboard pier did not exist - that road appears to all be fill that moved the shoreline out so that back in the day, the station was essentially part of the pier. Other pictures show trains on the pier (the original pier, not today's Steel Pier) but I think also from the original station (the current station was built in 1911). The details in the book are sketchy but imply that from 1867 to 1875, the Lake George branch was essentially the main line of the Rensselaer and Saratoga (later acquired by the D&H) with a trip from Albany to Montreal being train to Lake George, lake steamer to Baldwin, stagecoach to Ticonderoga, and then a Lake Champlain steamer further north (and probably back to land transportation for the last distance to Montreal). But in 1875, the current D&H main through Whitehall and along Lake Champlain opened.
The bike path to Glens Falls is mostly on the D&H ROW although at points it moves over to the parallel old (abandoned in the '30s I think) interurban right-of-way. The power lines mark the interurban ROW (as they do in many, many places in the U.S.), not the D&H ROW. I have no idea what the interurban did when it got to Lake George Village. As you get towards Glens Falls, the ROW has been severed by a golf course and the bike path takes to roads. And of course it's severed at Lake George by the miniature golf course and by the battlefield park the OP was at. As mentioned above, one bridge is still intact and is used by the bike path to cross route 9L south of the Village. I believe it is still lettered for the D&H.
The picture of the triple-header the OP referenced I suspect is the same picture in the book I have (credited as "From an original glass plate by Fred Thatcher, 'Steamer Dock at Lake George Village'"). The steamboat in the background of the photo is the Horicon II which operated from 1911 to 1939 helping to date the photo.