Most IHC steam engines I have put decoders into never required opening the tender at all -- I use a TCS M-1 decoder and put it in the boiler.
Anyway, to open up a boiler on practically all IHC models, you must first remove the pony truck. Under that, there is a long screw that goes up through the cylinders and weight. After removing this screw and handrails from the front of the engine, lift up on the front to separate it from the frame. The back of the superstructure usually has two tabs at the back holding it down.
The boiler faceplate can be removed by gently prying and pulling forward on it. On some models I have had to remove this and untwist the headlight wires so the boiler front can be raised up.
Also, on practically every IHC steamer I have put a decoder into, the headlight is connected to the front set of drivers, and there is a long electrical contact strip running the length of the frame, back to the motor connections. In order to rewire the headlight, you have to cut the wires and splice onto them.
The biggest problem I have encountered is that there is no room to thread the wires beyond the weight to reach the back of the engine without removing the weight and cutting a groove into it, so I usually leave the leadlight connected to the front pickups, which means that it is always on and can't be controlled by the decoder.