It might be entirely folk legend. Remember how frustrated we were when that Lehndorff entry didn't name the soldier? Perhaps because once Fritz was King, a great many people claimed to have done something for him during the Küstrin year, and that's how it came about. Otoh, it's interesting that Manger and Lehndorff, who are two independent contemporary sources unaware of each other, have both heard specifically this blowing out/reigniting the candle story, which means there must have been some kind of Ur-tale that then spread, with apparantly the builders in Potsdam subsequently adapting it to declare that soldier had been Knobelsdorff.

Given that Fritz was demonstrably found of the Münchow family thereafter, I'm currently guessing your version is the most likely one and fits with other things they did for him.