Very good summary, but I think the Treaty was 1907, and it included the 11 map set which defined the border. There is little doubt in my mind that Captain Bertrand was deliberately tricky in diverting the border around Prah Vihear, but there was no-one watching him. Not only did the Siamese not send surveyors they didn't even send observers, and they accepted the 11 maps drawn up by the French and issued them to foreign governments and local officials (all from ICJ Court documents). Then before the ICJ in 1962 they tried to claim that (i) they hadn't agreed to the maps, even though they were attached to the Treaty they had signed, or (ii) they didn't understand that the line had been drawn in the wrong place by Captain Bertrand. Since it was 55 years later it is hardly surprising that the ICJ basically said, you have left it a bit late to claim the maps are wrong.
Now we are left with the incredible difficulty of defining an inernational border. To do so on paper is quite easy, by darwing a line such as the 49th parallel, but then it is quite hard to define that line on the ground. Today we can do it instantly with GPS, but back in 1907 when they only had theodolites it was very much more difficult. That was why the Joint Border Commission opted to change it from a line on a map to follow geographical features such as a river or a mountain ridge. That is much easier to identify on the ground, but very difficult to draw on a map, and also rivers do move with time. Throw into the mix a tricky French surveyor being left to his own devices by the Siamese surveyors, and you have the current mess. When there are two parties, and one says "the border is here", while the other says "No it isn't, it's here", there is unlikely to be any agreement, which is why someone like the ICJ is necessary. I am quite sure Thailand would have accepted the ICJ decision if it had been in their favour, but it wasn't, so they didn't. Just bad losers.
In the end, an agreed shared zone for the land which has changed hands over rmany centuries may be the best solution, but the nationalists probably won't allow it.