I think several things got mixed up. If there are no out of gamut colors in the srgb space, there should be no difference in rendering between any working color spaces greater than srgb.
The difference you see is I believe due to different default renderings applied for the classic rendering and wide gamut rendering. The classic one will use the generic camera style. The wide gamut one will use the neutral style as default.
Can you apply the Adobe flat profile to the classic rendering? It should look identical.
About the magenta color cast in the soft proofed image, that looks more like a bug? There should be no change in colors if no colors are out of gamut.