You are correct, the 10 would fall out of the Gp100 holster upside down after it has been worn a while. I'm sure the Gp100 will fall out to after a while too.
I am mainly trying to be able to build holsters for the 4" K frame. The 686 I am not to worried about because I can buy a bluegun for it or use the GP100. With the K frame it is harder to get the variations. The 4" K frame bluegun will cover the 10/64,13/65 with a heavy barrel, but then some of the 10/64 have pencil barrels. The 15/67 has adjustable sights and the 19/66 has adjustable sights with a half underlug so there are some big differences on a molded and pressed holster.
I would like a dummy gun for each model because like you I make holsters for the specific gun. I am trying to figure out if I am being to specific. My main problem is I have not been able to find a 15 dummy gun or a 19 dummy gun.