I am going to go out on a limb here and offer an opinion on something that I didn't actually witness. It SOUNDS to me like the areas that were spot annealed were allowed to cool too rapidly. After being heated they need to cool very, very slowly. Just removing the heat source and allowing it to sit may be allowing the area to remain in a hardened state. The cross sections where the holes are located are thin. The few times that I have had to do this I just used the torch and kept the heat on and gradually backed it off. This can be tricky if you are not careful and have a good bit of experience with a torch. Something else that may work a little easier and more controlled is to fill a metal coffee can with dry sand, heat it in the oven to about 350 deg. or so and get the sand thoroughly heated. Heat your receiver spots and immediately bury your receiver in the sand. Let it cool until the sand is cool to the touch. Having said this I will also say that I have never even seen a Siamese mauser action. I have had to spot anneal a few holes in other mauser actions and I think either method will work well for you.
I also have a drill that I bought to drill out a broken high speed steel, 6-48, tap. It is tiny. It cost over $20. I looks like it would break if you dropped it on the floor. It drilled through the tap like it was made of butter. I was amazed to say the least. I bought it from Travers Tool Co. but that is all I can remember about it. Just be careful, earlier someone posted that he used only carbon steel taps for easier removal in case it breaks. Someone else suggested using a carbide tap. These are extreme opposites in the hardness and brittleness areas. When you break that carbide tap off in the receiver thing are going downhill fast. There are only a few ways to get it out and you ain't gonna like none of them. Be careful and good luck!