Maciek.
I just had a brief look at Community Forum and are astonished about what people know and discuss. I agree there are a lot of details which don`t match in all the stories on U 864. The basic is the U 864 had a schnorchel installed just before she left the yard in Germany. They spent 4 days sailing to Norway included grounding in Farsund area. She was in the Bergen submarine pen during an air raid where dambusters were used, one penetrated the roof. Just after the bergenrepair they decided to train the crew in schnorcheling prior to the departure to Japan, thus inexperienced crew doing exercises on schnorchelling off the Norwegian coast. I agree that the schnorchel mast is the first thing to be spotted rather than periscope. The theory on seasickness and zig zacking while schnorchelling I have never heard of. The talks and theories about supercharger is strange and not worth to comment.
I guess U 864 was exercising the inexperienced crew on schnorchelling, the noise picked up by the sonar of HMS Venturer, probably not experienced in diesel sound while training schnorchelling ( they would not use the engine with a failure). They had decided to return to Bergen for engine repair. The supercharger should not be used while schnorchelling. I believe they had stopped schnorchelling, still submerged switched to E-motors, took the usual sweep by the sonar and realised they were spotted started zigzagging heard the torpedoes were coming and crash dived deep. This is my theory, but take it with a grain of salt, I have not read any reports on the sinking.
Tore